and, in the case of ciliated cells, the cilia are continuous with the intracellular network. So that Klein's view of the nucleus is that it consists of a central, more complicated portion of the general intra-cellular network, shut off from the rest by the formation of an investing membrane. Professor Flemming 13 denies the existence of a connection between the intra-nuclear and the intra-cellular networks; and Herr Schleicher 14 considers that in cartilage-cells, at any rate, there is not even a true network in the nucleus, but merely a number of independent filaments, rods, and granules, which put on the appearance of one.

There is also a difference of opinion with regard to the nucleoli, the usually highly refracting bodies which occur in nuclei, one or more to each, and which, prior to the observations we are recording, were the only points of structure made out in nuclei beyond the possible existence of a membrane. Professor Flemming believes that these have a real existence-that they are, in fact, granules lying in the meshes of the intra-nuclear network, and of a different substance from the latter. Dr. Klein, on the other hand, considers them to be mere local thickenings of filaments, or to result from the shrinking and fusion of a part of the network. The latter view is favoured by their great inconstancy, and by the fact that they are often by no means evident in the fresh condition.

The foregoing observations all apply to nuclei in the quiescent condition; the nucleus of dividing cells has also been the subject of careful study, both in animal and vegetable cells. The main steps in the process, as given by Auerbach, Strasburger, and other earlier observers, are shortly as follows. 15 The nucleus elongates until it has assumed a spindle shape, and exhibits a longitudinal striation, the striæ converging at its extremities, and gradually diverging as they reach its equator, where is finally seen a disc-like structure placed at right angles to the long axis of the nucleus, and composed of short rods continuous with the striæ. This nuclear disc splits into two, its rods becoming drawn out in the centre, while their still thickened ends travel outwards along the striæ of the nucleus, until they reach one of its poles; in this way there is formed at each extremity of the spindle an accumulation of substance. These two accumulations, assuming gradually a more defined shape, become the nuclei of the two daughter cells into which the mother cell is dividing. A second disc then appears in the equator of the nucleus by central thickenings of its striæ; this cell-disc also splits into two, the plane of junction. between its two segments marking the division plane between the

13 Beiträge zur Kenntniss der Zelle und ihrer Lebenserscheinungen.' Archir

f. micr. Anat., xvi. Band, 2 Heft, December 1878.

14 Die Knorpelzelltheilung.'

cember 1878.

15 See Priestley, loc. cit.

Archiv f. micr. Anat., xvi. Band, 2 Heft, De

new daughter cells. Often, during this process, the protoplasm of the cell takes on a radiate arrangement round the poles of the spindle, producing the appearance of a sort of double star.

These appearances in the dividing nucleus were made out in various vegetable cells and in the ova of many animals; the relation between them and those described above in the quiescent nucleus seems by no means clear, but special attention has been directed to this point by Flemming 16 and by Schleicher," whose results in the case of the nuclei of epithelium, cartilage, &c., show a very close agreement in the main features of the process, although they differ considerably in detail. Both have directly observed the division process in living cells. Flemming has also availed himself extensively of various preservative and staining fluids, and it is in his very beautiful figures of these hardened and coloured nuclei that the chief discrepancies are observable between his results and those of Schleicher. It will be seen that their results differ in some particulars from those of former observers.

The first step, preparatory to cleavage, is a disintegration of the nuclear membrane, fragments of which seem to become mingled with the other constituents of the nucleus. Then the filaments of the intranuclear network (or the rods, filaments, and granules, according to Schleicher) assume a very lively condition of motility, and go through an extraordinary series of arrangements and re-arrangements, producing figures now star-like, now wreath-like, now more irregular, but, according to Flemming, taking place in a definite order. The filaments then assume an almost parallel disposition--forming the socalled cell-plate or equatorial plate—and then, diverging somewhat towards the equator of the nucleus, and converging at its two poles, produce the spindle form already described. The remarkable series of bacterioid movements by which these changes are produced is called by Schleicher karyokinesis:' the process is so vigorous that the nucleus of cartilage-cells, freed from its membrane, travels now to one pole of the cell, now to the other, now contracts and now enlarges to such an extent as to fill up nearly the whole cell.

After the assumption of the spindle form, the formation of the new nuclei begins, according to Schleicher, by a fusion of the converging ends of the filaments-the ends, that is, turned towards the poles of the spindle-while at the same time a separation of the fibres takes place along its equatorial plane. The young nuclei are, therefore, tolerably solid at first, but afterwards split up into filaments, rods, and granules, some of which, at the periphery of the nucleus, curve round, unite with one another, and form a new nuclear membrane. According to Flemming's observations, which are partly borne out by Schleicher's, the daughter nuclei go through the same series of

changes, but in inverse order, as were undergone by the mother nucleus in the various stages of karyokinesis.

There is one observation of Schleicher's which has an interesting bearing on Klein's view of the connection between the intra-cellular and intra-nuclear networks. He observed, in certain cases, filaments in the cell-protoplasm which seemed to become continuous with the nuclear filaments during karyokinesis. Moreover, these cell-filaments had an intimate connection with the capsule of the cell, leading to the opinion that they were split off from its internal surface by a process of delamination,

Closely connected with these researches on nuclei are the numerous observations lately made on the phenomena which accompany the maturation and impregnation of the animal ovum. The ripe egg consists essentially, in all animals, of a vitellus or protoplasmic cellbody containing usually a greater or less quantity of food material in the shape of yolk-granules, of a surrounding vitelline membrane or cell-envelope, and of an enclosed cell-nucleus or germinal vesicle, which latter is contained, like the nuclei of which we have been speaking, in a distinct membrane, and contains one or more nucleoli or germinal spots, as well as a delicate network of protoplasmic filaments.

It has long been known that, prior to impregnation, the germinal vesicle undergoes important changes, but what is the nature of these, what the precise nature of the influence exerted by the spermatozoon or spermatozoa effecting impregnation, and what the mode of origin of the nucleus of the impregnated egg or first segmentation nucleus, has only recently been made out. The observers to whom we owe this important and difficult piece of work are chiefly Auerbach, Strasburger, Van Beneden, Bütschli, Oscar Hertwig, and Fol; a résumé of their observations was published last year by Mr. Balfour, 18 who has himself made important contributions to the subject. Since the publication of this résumé, the most noteworthy papers are those of Oscar Hertwig 19 and of Calberla,20 the former of whom has studied the eggs of molluscs, worms, and echinoderms, the latter those of the lamprey.

With regard to the fate of the germinal vesicle, the most complete observations are those of Hertwig, whose figures, drawn very largely from the living objects, seem to leave no doubt about the main steps of the process. The vesicle travels from the centre to the surface of the egg, its membrane undergoes disintegration, its nu

18On the Phenomena accompanying the Maturation and Impregnation of the Ovum.' Quart. Journ. of Microscopical Science, April 1878.

19 Beiträge zur Kenntniss der Bildung, Befruchtung und Theilung des thierischen Eies. Dritter Theil, I. Abschnitt.' Morphol. Jahrb., iv. Bd., 1 Heft; and 'JI. Abschnitt,' ib. 2 Heft, 1878.

2. Der Befructhungsvorgang beim Ei von Petromyzon Planeri.' Zeits. f. wiss. Zool., xxx. Bund, 3 Heft, 1878.

cleolus disappears as such, and it is converted into a spindle-shaped body, with the usual delicate striations, and with the star-like radiation of granules from its poles. A prominence is then formed on the surface of the vitellus, into which one end of the spindle passes; the spindle itself undergoes division in the usual manner, one segment being left in the egg, the other in the prominence, which then separates itself as the first of the polar bodies' or 'directive corpuscles'-small structures now proved by their mode of formation to be true cells, which lie between the egg itself and the vitelline membrane, occupying a constant position throughout the early stages of development. The likeness between this process and those mentioned above as characteristic of ordinary cell-division will be at once apparent.

The next step is the formation, in the same manner, of a second polar body, the spindle-shaped nucleus, as before, dividing into two, one for the polar cell, the other for the egg itself. The latter portion undergoes a change, being converted into a more or less rounded. body with a radial striation of granules around it, the female pronucleus (Eikern), which gradually travels to the centre of the egg. In the meantime, a (in most cases) single spermatozoon has made its way through the vitelline membrane into the vitellus, and, its tail being lost or fused with the vitellus, its head has been converted into a body closely resembling the female pronucleus, and known as the male pronucleus (Spermakern). This travels towards the female pronucleus, and completely fuses with it, the first segmentation nucleus being the result of the fusion.

As Mr. Balfour remarks, the head of a spermatozoon is, in all probability, the modified nucleus of a spermatic cell, so that the process of impregnation consists in the conjugation of two nuclei. Moreover, both ova and spermatic cells are developed in the embryo from certain cells of undetermined sex known as primitive ova,21 so that this conjugation is a union of morphologically identical structures.

21 See Balfour, 'On the Structure and Development of the Vertebrate Ovary.' Quart. Journ. of Microscopical Science, October 1878.

PROBABILITY AS THE GUIDE OF
CONDUCT.

THE doctrine of Bishop Butler, in the Introduction to his Analogy, with regard to probable evidence, lies at the root of his entire argument; for by the analogy which he seeks to establish between natural religion and that which is revealed, he does not pretend to supply a demonstrative proof of Christianity, but only such a kind and such an amount of presumptions in its favour as to bind human beings at the least to take its claims into their serious consideration. This, he urges, they must do, provided only they mean to act with regard to it upon those principles, which, in all other matters, are regarded as the principles of common sense. It is therefore essential to his purpose to show what are the obligations which, as inferred from the universal practice of men, probable or presumptive evidence may entail.

But indeed the subject-matter of this Introduction has yet a far wider scope. It embraces the rule of just proceeding, not only in regard to the examination of the pretensions of Christianity, but also in regard to the whole conduct of life. The former question, great as it is, has no practical existence for the vast majority, whether of the Christian world, or of the world beyond the precinct of the Christian profession. It is only relevant and material (except as an exercise of sound philosophy) to three descriptions of persons; those whom the Gospel for the first time solicits; those who have fallen away from it; and those who are in doubt concerning its foundation. Again, there are portions of these classes, to whose states of mind other modes of address may be more suitable. But every Christian, and indeed every man owning any kind of moral obligation, who may once enter upon any speculation concerning the grounds which lead men to act, or to refrain from acting, is concerned in the highest degree with the subject that Bishop Butler has opened incidentally for the sake of its relation to his own immediate purpose.

The proposition of Bishop Butler, that probability is the guide of life, is not one invented for the purposes of his argument, nor held by believers alone. Voltaire has used nearly the same words :

Presque toute la vie humaine roule sur des probabilités. Tout ce qui n'est pas démontré aux yeux, ou reconnu pour vrai par les parties évidemment intéressées