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embryo at the present day is a question of very great dif ficulty.

It can hardly be looked upon as a cavity comparable to the segmentation cavity of Amphioxus or Amphibia, or the Lamprey or Ganoids, for it has a fate different from that in any of those animals. The fate of the segmentation cavity of the above-mentioned animals is to disappear and take no part in the formation of the cavities of the adult.

The cavity of the blastodermic vesicle whose formation we are about to discuss never disappears, never, at any rate, in the rabbit, as part of it remains as the cavity of the alimentary canal of the adult. Part of it undoubtedly is comparable to the archenteron; possibly all of it is, for it becomes the gut-cavity of the adult.

The first cause which produces the cleft that subsequently enlarges into the cavity of the blastodermic vesicle may be a more active growth of the outer layer of cells. Undoubtedly there is after this time a more active growth of the cells of the outer layer. They increase much more rapidly than the cells of the inner mass.

It is not easy to explain why the energy which up to a certain time causes a solid morula should do so no longer. Why should not the morula steadily increase in size, but be still a morula? Although I believe that the subsequent vast increase in size of the blastodermic vesicle cavity is due to the diffusion inwards of fluid derived from the uterus, still this can hardly be the cause of the first starting of a cleft as in fig. 21. This seems to be best explained by the assumption of an increase in rate of growth of the outer cells over the inner cells of the morula. That such an increase does exist the following table provides evidence.

In a median section through an embryo:

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This, though rather a rough method of investigation, shows sufficiently well that there is a proportionally greater rapidity of increase of the cells on the outside over that of the inner cells.

Growth of the embryo must surely be dependent upon nourishment from without, when the bulk of the mass increases as it does from the 70th hour. It is the cells upon the outside of the morula which are in the most favourable position for the acquisition of such nourishment from the fluids of the uterus or Fallopian tube. No doubt during the early stages of segmentation the energy of division is derived from the nutriment -yolk, &c.-contained within the ovum itself. As this becomes used up the embryo will become more dependent upon external sources. This will mean that the externally placed segments will become more favourably placed for growth than the internally placed segments. May not this gradual exhaustion of intrinsic nutriment be a determining factor in the cessation of the increase of the embryo as a morula and causation of the first commencement of a cavity?

This may give rise to the first cleft, but in itself it can hardly account for the large increase in size of the blastodermic vesicle. The cells are so very delicate it is hardly conceivable that they could cause the great expansion of the very tough albuminous wall. It seems far more likely that the force which causes the expansion is due to an osmotic current being more rapid inwards than outwards, either simple or more probably assisted by the vital activity of certain cells of the embryo, as is supposed in the case of diffusion in intestine, and suggested by Heape in connection with the mole. Or the diffusion process may be a simple physical process, but the nature of the liquid after entering the cavity may be so changed by the activity of the cells as to render its diffusibility less when once inside than before its entrance from the uterus.

However this may be, the most noticeable fact of the development of the rabbit embryo during the fourth day is the commencement and enlargement of the cavity of the blastodermic vesicle.

Up to the moment of the beginning of the cavity there does not appear to be any pressure upon the walls of the embryo by zona radiata and albumen layer; in fact, the embryo may often be found to be slightly retracted from the zona radiata in the fresh state.

The diameter of the cavity within the zona radiata is, in the unsegmented ovum, about 0·11 mm., and up to the time that the cleft appears it has not greatly enlarged, measuring only about 0.12 mm.

On the appearance of the cleft the cells of the outer layer become pressed hard against the zona radiata and flattened, but no great increase in diameter of the outer border of the zona radiata is as yet recognisable, although the thickness of the zona radiata is diminished, apparently being of a compressible nature, and therefore becomes compressed between the pressure from within the blastodermic vesicle and the resistance afforded by the tough albumen layer from without. At this time, being in the very firm lower part of the Fallopian tube, the resistance afforded by the albumen layer is very likely aided by the walls of the Fallopian tube itself. In this condition the embryo usually passes into the uterus, although sometimes specimens may be found in the uterus (but very rarely) in which no cleft has as yet appeared. The embryos, I think, pass rather suddenly through the last 4 to 6 mm. of the Fallopian tube at some time between the 75th and 80th hours after coition.

No increase takes place in the thickness of the albuminous layer after entering the uterus, but by the stretching of it caused by the expansion of the blastodermic vesicle it rapidly thins. The zona radiata thins so much as to be hardly perceptible by the end of the fourth day.

Van Beneden's figures, 5, 8, 9, 10, 11, on pl. iv of his paper, taken from optical sections, represent very well the appearances presented by the embryos during these changes.

As van Beneden's figures represent optical sections, it is, I think, advisable to give a series of figures drawn from real sections, as none have hitherto been published.

Figs. 20 to 34 are all camera drawings, and each is magnified 465 times.

Figs. 20 to 25 show sections through the whole embryo, the subsequent figures through the embryonic disc, or a part of it only.

Fig. 20, and also fig. 16, are sections of silver nitrate preparations.

In both cases there is no difference between the cells at the surface and those towards the centre as regards their colouring or nuclei. Those inside are certainly more compressed than those on the surface. So also it frequently happens that a large segment may be so placed that although the greater part of it may be said to belong to the "inner mass," yet a small part of it may appear on the surface, as x. in fig. 20.

If such a specimen is examined in optical section, this individual cell might very likely give rise to such an appearance as van Beneden describes, and lead to the idea of an inner mass partially surrounded by an outer layer. The difference in colour which van Beneden describes is totally absent in real sections, and I can find no greater opacity of the inner mass in optical section than what can be equally well explained by the greater thickness through which the light must of necessity pass in viewing a sphere in optical sections. I am not able to offer any explanation of the condition figured by van Beneden in his second figure; I can only say that I have have not been able to find it.

Fig. 21 is a section through an unusually large specimen. This specimen was preserved in Perenyi. This specimen I believe shows the earliest stage in the formation of the cavity of the blastodermic vesicle (C. BL.).

There is as yet no evidence of an internal hydrostatic pressure, but the outer cells form a compact layer, and seem at one point to be as it were lifted away from the inner cells, leaving a slight cleft (C. BL.).

In fig. 22 this cleft has increased considerably. It must be noticed that it does not extend through more than about 240°.

Through the remaining 120° the inner mass is still as much part of the wall of the vesicle as the outer layer of cells.

In fig. 23 the cavity shows a further increase in size. The outer layer of cells (O. L.) now show signs of having become stretched, due, I believe, to the rapidly increasing hydrostatic pressure within the blastodermic vesicle. The walls of the vesicle are now, while living, firmly applied to the zona radiata and albumen layer. The space shown between the vesicle and the zone in the figure is due to reagents. The same remark applies to all the figures from the 22nd onwards. I have not been able to distinguish sharp lines of division between the outer layer cells in section after the commencement of the blastodermic vesicle cavity. In surface view there are certain lines of divisions, which, as van Beneden has shown, are very clearly brought out by silver nitrate.

In this fig. 23, which I believe to be a median section, there is a fairly clearly defined line marking the inner mass from the outer layer, which is less perceptible in fig. 22, and which becomes very distinct in later stages, such as in fig. 28.

It is now possible to distinguish clearly an inner mass as separate from the outer layer. This separation seems to be caused simply by the tension being more acute in the outermost of the segments of the mass I. M. in fig. 22, causing these segments to be more stretched than the remainder, because they are more directly united with the already separated and stretched cells O. L.

I

Lines of division between the segments of the inner mass can usually be found, but varying greatly in definition. cannot say to what extent these several segments may be really divided. The inner mass is certainly connected in some way with the outer layer, but whether by direct protoplasmic union I am quite unable to say.

Fig. 24 presents no new characters excepting a tendency to flattening of the inner mass. In figs. 25, 26, 27, the inner mass is flattened out still more, so that it presents a lenticular form in section instead of a circular outline as it did in figs. 22 and 23. How does this change in form come about?

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