embryos (P. 64); and a similar obscure account is given of the germs of the "Altrices." The general laws of development, as well as the fact, already stated, that in the similar process among the Campanulate polypes the vesicula Purkinji was detected, lead to the conclusion that each gene. ration springs from true germinal vesicles; and, as imperfect animals are never fertile, these vesicles must all have been derived from the parent distoma; but how the germs of the successive broods are placed as regards each other, we have at present no means of determining. In all cases, however, it is certain that the embryos are lodged when they first appear in the posterior part of the body between the two lateral processes existing there; "it cannot be doubted," adds Steenstrup, "that the germs are always collected in that situation as in a distinct organ, (uterus?)" As to the mode of birth, it would appear that the Cercaria quit their "nurses" at a particular depression under what is called the collar, where two apertures are supposed to be placed. An examination of the plates (more particularly Tab. 2, fig. 3, ah, fig. 5, a-h) confirms the conclusion that, however much the process may deviate from ordinary generation, there is no departure from the fundamental laws.

It has always been a difficult point to comprehend how the Entozoa, which have their habitat in the very substance of organs, the Trichina spiralis for example in muscle, effect their entrance. Although this gene. ral question cannot be resolved by the proceedings of the Cercariæ infesting aquatic animals, still the mode in which those creatures bury themselves in the skin of the snail on assuming the pupa state, enable us at all events to understand, as the author remarks, how they can enter the nobler internal organs. In this instance it is probable that the instrument for effecting the passage, consists of the circlet of spines on the collar around the oral orifice, since these are found on the Cercariæ, even after they have escaped from the pupa-case, but are cast off after the Distoma has gained its nidus. It is further interesting to remark that, as the Cercaria of water-snails are proved to undergo their metamorphosis in the interior of the abdominal cavity as well as on the outer surface, there is no difficulty in perceiving that, in higher animals, as mammalia, altrices, or their progeny of larvæ, might be introduced into the alimentary canal, and thence, to borrow a common expression, work their way into the paren chyma of the organs.

The observations of M. Steenstrup respecting the generation and de velopment of Entozoa in general, are of such deep interest that no apology is required for laying them before our readers.

"When a metamorphosis occurs so extensively, or is even universal in one division of the entozoa, the question naturally arises, whether, with respect to the other divisions of that class, it is probable that a similar transformation is effected by a single metamorphosis, or whether even they probably do not attain their development through a series of alternate generations, and exist at first externally to the organisms with which their life is afterwards inseparably connected. Although I cannot commit myself to a precise reply to this query, which is rather beside the especial object of these pages, yet I cannot help giving a few hints on the subject. The Nematoidea, which in their adult state often pass from one individual to another, also probably penetrate from without and as embryos, the organism they infest; they do not appear to undergo a true metamorphosis, but to change their skin; I am also unacquainted with any observation which would

1846.]

REPRODUCTION OF ENTOZOA.

6

31

justify the supposition, that there is in this group any fostering of the young by precedent generations, unless the genus Sphærularia, a parasite of the Hymenop tera, which Siebold refers to this division of entozoa, be a nurse;' at least it much resembles one, appears to be nearly powerless, to perform spontaneous movements, and contains a numerous smooth-skinned progeny, which move about very actively within their parent, but bear no resemblance to it. The cystic entozoa, on the contrary, betray in many ways that they are a nursing' generation, and especially in the singular circumstance of their being frequently inclosed like boxes one within the other. Probably the full-grown animal of this division is quite unknown; and it is not unlikely that, in course of time, it may happen with them as it has with the whole division of the asexual' Trematoda of Siebold, viz., Cercaria, Leucochloridium, &c., that they must be rejected from the system as being earlier forms of development, or earlier generations of other animals. The Echinorhynchi present several phenomena which are inte resting with regard to our object, viz., the remarkable incubation or breeding which takes place between the skin and the viscera, and the inclusion and incipi ent development of the ova in the so-called loose ovaries,' during the continued growth of the latter. I must confess, that I look upon these oval bodies much rather as individuals which will never quit the parent animal, than as 'ovaries;' and till their true nature is known, I shall regard them as such, and consequently consider most of the Echinorhynchi hitherto known, as nurses.' It is confessedly uncertain whether the Echinorhynchi spend part of their life externally to the organism which they inhabit as full-grown animals, or not; it is however very probable they do so, as the embryo attains no real development in the ova, so long as these are in the Echinorhynchus; and the ova are met with in the mucus of the stomach and in the excrements, by thousands, in the same condi tion, so that the development of the young in the ova and their escape from them, certainly occurs very long after the ova have reached the water." P. 101.

"Lastly, with respect to the metamorphosis of the Cestoid worms, we have an example of it presented to us in Miescher's interesting memoir on the forms which the genus Tetrarhynchus passes through; but which also in fact includes nearly all we know about them; whilst the great work of Professor Eschricht, upon the genus Bothriocephalus of this family, has given an entirely new view of those animals, constituted of almost innumerable joints, so that they are to be regarded, according to his anatomical investigations, not as single but as compound animals, viz., compound Trematoda or flukes, so that each joint is to be compared with a distoma, a view of their nature to which Baer had some time before alluded, and which had also struck Creplin and Mehlis, on account of the resemblance of the sexual organs, but which is now for the first time entitled to the greatest attention, having been rendered probable by so many anatomical researches. However, I cannot, for my part, entirely coincide in this view of the Cestoid family of worms, for in whatever way the joints and their reciprocal connection are considered, compound animals are presented, whose construction is entirely different from that of all other animals. The tapeworm is certainly not a single individual, but consists of several; that is, it is constituted of the head, which is an animal, and of the progeny derived from it. This view is much supported; it is even proved, by the fact that the offspring (joints) in a state of progressive development, never actually become animals similar to that from which they spring, (the cephalic joint,) which alone remains dissimilar to all the rest, never acquires any developed sexual organs, and consequently never generates any ova, which the others produce in great abundance; whilst the cephalic joint is furnished with a mouth and suctorial acetabula, proceeds from an egg, and is the animal upon which the development of all the rest depends, and is thus in fact one of those animals to which we have in the preceding pages given the name of nurse.' If this view be correct, which time will determine, it will be seen that the Bothriocephali, just as all the other animals

mentioned in this Essay, which are developed through alternating generations, present quite another and more significant resemblance to plants, than that set up by Professor Eschricht. The individuals which are fostered by, and appear to proceed from, the head by the so-called transverse division, thus attain such a degree of perfection, that the ova are fully formed in them even before they become detached from the nursing animal;' and, as defensive cases for the ova, they are passed in the natural way from the animal which they have infested, in order probably to reach, eventually, another similar animal, under another form, and as other individuals." P. 104.

6

In concluding our notice of this remarkable work, we are anxious to point to some of the more important results which it seems to establish. And, first of all, it will not be superfluous to state, as corroborative of the author's views, that many of the facts upon which they repose have been ascertained by other and earlier investigators, and are acknowledged to be 'well founded; thus, whatever doubts may be entertained, they must have reference not to the phenomena themselves, but to the way in which these are to be interpreted. They, the phenomena, have usually been regarded as instances merely of metamorphosis or transformation; but, as M. Steenstrup justly observes, the essential objection that a metamorphosis can only imply changes which occur in the same individual, has been overlooked; and, further, that when from an individual other individuals originate, something more than a metamorphosis is concerned.

"Thus, it is quite erroneous to term a Scyphistoma the larval condition of Medusa aurita, since Scyphistoma never becomes a medusa, but is the quasi mother of some scores of them. Sars and Lovén have taken a more correct view of the relation in which these creatures stand with regard to each other, seeing, in the development of the Medusa and Campanularia a series of generations undergoing metamorphosis. It is of the more essential importance that the distinction between an alternation of generation and a metamorphosis should be understood, because a metamorphosis may readily occur in one or other of the alternating generations themselves, as we see, for example, in the Distomata and Aphides." P. 6.

In the next place, it is to be remarked that, however bizarre and exceptional the phenomena in question may seem to be, they in reality fall within the general laws of development, of the fixity and universality of which, when correctly viewed, they offer a most striking example. It is here, as in all other sciences, perceived, that the admission of exceptions is indicative of imperfect knowledge and of limited observation; for, in proportion as facts multiply and afford the materials for induction, what had hitherto seemed to be isolated and exceptional instances are found to form but a part of a whole series of phenomena, all conformable to fixed and undeviating laws. In the case before us, it would have been a real exception, if the imperfect animals, forming the intervening generations, had been prolific; but it has been already shown that they have no ovaries, (or more probably that they possess those organs in an undeveloped state,) and we may, with the author, further assume, "that the nursing' individuals are never themselves gemmiparous, but that they are born with germs in the organs (uteri) in which the embryos are afterwards nourished."

In connection with this way of viewing the subject, it is not one of the least interesting results, that the curious economy of the bee and wasp, of

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GENERAL RESULTS.

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ants and termites, which has up to the present time been regarded as a thing sui generis, is shown to be but a part of a mode of generation extensively prevailing among the lower animals, and which is itself only a modification of, and not a deviation from, the normal female action-the nutrition of the germ.

We have already pointed out, what cannot have escaped the notice of our readers, that many instances of what have hitherto been regarded as gemmiparous and fissiparous generation, will probably be shown to belong to the system of alternate generation. Nor is it attaching too much importance to these researches to affirm, that they will throw considerable light upon some of the still obscure parts of the reproductive process, and, among others, on equivocal generation.

Among what the author properly styles secondary results, are some highly important facts: such as "that the Cercaria are larvæ of entozoa, of the genus Distoma, and in fact of those species which inhabit the interior of fresh-water snails (in the liver, &c.;) that the entozoa pass part of their existence in a state of freedom in the water external to the snail, which they afterwards inhabit, and that they re-enter them from without ; that whole established divisions of families of animals must be abolished, since they include only undeveloped forms, or forms which bear the same relation to the true and perfect form of the species, that the workers' among ants and bees bear to the fertile female of those insects; and, finally, that several forms which have been considered as of different species and genera are seen to be stages in the development of one and the same animal." To this enumeration must also be added the important light which is thrown by these inquiries into the character of what have been regarded, either as asexual or hermaphrodite animals, but which, at all events in many instances, are nothing else than imperfectly developed females.

M. Steenstrup thus philosophically sums up his researches :

"I conclude with the remark, that, inasmuch as in the system of nursing,' the whole advancement of the welfare of the young is effected only by a still and peaceful organic activity, is only a function of the vegetative life of the individual, so also all those forms of animals in whose development the nursing' system obtains, actually remind us of the propagation and vital cycle of plants. For it is peculiar to plants and as it were their special characteristic, that the germ, the primordial individual in the vegetation or seed, is competent to produce individuals which are again capable of producing seeds or individuals of the primary form or that to which the plant owed its origin, only by the intervention of a whole series of generations. It is certainly the great triumph of Morphology, that it is able to show how the plant or tree (that colony of individuals arranged in accordance with a simple vegetative principle, or fundamental law,) unfolds itself through a frequently long succession of generations, into individuals, becoming constantly more and more perfect, until, after the immediately precedent generation, it appears as Calyx and Corolla with perfect male and female individuals; stamens and pistils (so that even in the vegetable kingdom the grosser hermaphroditism does not obtain, which is still supposed to take place in the animal,) and after, the fructification brings forth seed, which again goes through the same course. It is this great and significant resemblance to the vegetable kingdom, which in my opinion is presented by the entozoa, and all nurse' generations, and to which I have alluded in the preceding Essay; I might almost say, that the condition of continued dependence incidental to the animal life, is, to a No. 105. 3

certain extent, one of less perfection than that which is presented in the progressive elevation in development effected by the agency of the vegetable life." P. 115.

We have extended this article beyond the limits originally designed for it, having ourselves derived the greatest satisfaction, and we may add instruction, from perusing the deeply scientific and profound work of M. Steenstrup; and we should not do justice to our own sentiments, if we did not congratulate the Ray Society, and that excellent observer Mr. Busk, on having been the medium of making known to the English public, a series of researches equally interesting to the physiologist, the naturalist, and the pathologist.

ON DISORDERS OF THE CEREBRAL CIRCULATION, and ON THE CONNECTION BETWEEN AFFECTIONS OF THE BRAIN AND DISEASES OF THE HEART. By George Burrows, M.D. 8vo. pp. 280-Plates. London, 1846.

THE substance of this work has already been made known to a portion of the profession in the Lumleian Lectures delivered by the author in 1813 and 1844; but the subject treated of is one of such high importance, and the conclusions arrived at must, if admitted as sound, enforce so great a modification in the management of a large class of dangerous diseases, that an extended publication and criticism of the doctrines advanced is called for.

The emancipation of the mind from the despotic influence of great names is always a difficult and sometimes a very delicate task. There prevails so natural a feeling of veneration and gratitude for those who have preceded us in the paths of laborious investigation, and so many of their dogmas and principles are supported with able reasoning, well devised experiment, or sagacious observation, and have firmly resisted attacks from all quarters, that when any young practitioner (and it is ordinarily such who undertake opposition to any prevalent opinion) ventures to suspect that some of these may be fallacious, he is apt to be met with sneers or rebukes, and although he may have reason and truth on his side he will oftentimes fail in securing their reception. The difficulties, too, in breaking up a routine practice, however erroneously this may be based, can only be appreciated by those who have attentively observed the tenacity with which old prejudices and practices adhere even to well-informed persons, to say nothing of that preponderating class in all communities who continue to mechanically perform certain actions because they always have done so. When, however, we look back upon the history of medicine, and find that hardly a doctrine or theory once advocated by the ablest observers, holds its ground, and that what were once considered by acute men as axioms in practice, are now discarded as sheer fallacies by the merest tyro, we see the necessity of ever and anon re-examining the foundations of our medical creed, and submitting it to the reforms and improvements which the progress of science, the increased means of in