i894. SOME NEW BOOKS. 459 



however, does not show the ventral band of thick epithelium 

 which ultimately gives rise to the testes. From these follicle cells 

 short fertilising ducts grow out to the wall of the cloaca of each in- 

 dividual of the chain. Through these ducts fertilisation takes place, 

 and each salp of the chain thus comes to contain a fertilised egg, which 

 is to give rise to a solitary individual. In Fig. i is shown an individual 

 of the chain-salp of Salpa pinnata, with the developing embryo attached 

 to the wall of the cloaca. The egg and the embryo in its youngest 

 stages are really outside the wall of the cloaca, in one of the blood 

 sinuses of the chain-salp, but attached to the cloaca by the fertilising 

 duct which opens into the cloaca. As the embryo grows it pushes 

 into the wall of the cloaca, without, however, breaking through, and 

 so becomes suspended in a sac, which is really the in-pushed wall of 

 the cloaca, and which opens into the blood sinus. This sac remaining 

 open to the blood sinus of the chain-salp, expands considerably, and 

 becomes the placenta, by which the embryo continues to get its 

 nourishment from the blood of the chain. The edge of the cloaca, 

 round the place where the embryo is pushing in, grows in as a double 

 fold, the hollow between the layers of which is, of course, in continuity 

 with the blood space in which the embryo was formed. This double 

 fold comes to form a special sac, the amnion or embryo-sac, which 

 opens into the cloaca by a small pore, the space left by the edges of 

 the fold not completely meeting each other. Thus a horizontal sec- 

 tion cut through, such an en'.bryo as hangs into the cloaca in Fig. i, 

 would pass first through the outer wall of the amnion, next through 

 the blood space, thirdly through the inner wall of the amnion, fourthly 

 through the brood-pouch, which is part of the cloaca of the chain- 

 salpa, next through the epithelial capsule, then through the embryo 

 [see Fig. 3). The embryo-sac, however, does not increase in size, and 

 the growing embryo rapidly pushes itself through the small pore, and 

 comes to hang directly into the cloaca. 



Within these complicated membranes a process of a most 

 astonishing nature has been going on. The fertilised egg-cell was at 

 first surrounded by a layer of follicle-cells, which, themselves being 

 derived from the germinal cells, differ from the egg only in not being 

 actually developed into mature eggs and becoming fertilised (Fig. 4). 

 The egg-cell divides slowly, the follicle-cells divide rapidly, and 

 many of them push their way in towards the centre of the mass, and 

 actually push apart those cells which result from the division of the 

 egg-cell. During this time constant migration of nuclei takes place 

 from the ingrowing follicle-cells to the cells which have come from 

 the egg-cell. No doubt this is a process of nutrition, not very 

 different in kind from some known methods, although it is at least 

 curious to find nuclear matter being added from an extrinsic source 

 to the developing embryo. If it be remembered that most now believe 

 that the nucleus of a fertilised embryo is derived equally from the 

 nuclei of the male and female generative cells, that this nuclear 

 matter is the bearer of the inherited qualities of the embryo, and that 

 by subsequent assimilation and growth it gives rise to the entire 

 nuclear matter of the new organism, then it will be sufficiently 

 astonishing to find the nuclear matter of any cells serving simply as 

 nutritive material. But, if this is difficult to understand, what is to 

 be said of the other process, by which the embryo is, so to say, broken 

 in pieces during its development ? For what else is the separation 

 of the segmentation spheres by the intruding folhcle-cells ? Driesch 

 and Wilson have shown cases where the separation of the first two 

 spheres by artificial means resulted in the formation of twin embryos. 



