5S6 DEVELOPMENT OF RABBIT CHAP. 



Fertilisation and segmentation take place in the uterine 

 tubes (p. 540), and each oosperm is surrounded by a mem- 

 brane, the zona radiata, apparently formed from the follicle 

 cells, and a layer of albumen secreted from the oviduct. 

 The oosperm divides into two blastomeres (Fig. 154, A), and 

 then further divisions follow and result in the formation of a 

 mulberry-like mass (B), which, however, is not hollow, as ir 

 Amphioxus (Fig. 152), but consists of an outer layer en- 

 closing a solid mass of larger and more granular cells (C), 

 and at this stage it passes into the uterus. The outer layc 

 of cells then grows rapidly, so that a space appears and 

 gradually increases in size, between it and the inner ma 

 except at one point where the latter is attached to tr 

 former (D, E). This hollow ball is known as the blast ocyst 

 or blastodermic vesicle, and may be compared to the embryo 

 and yolk-sac of a bird, except that the latter contains a fluid 

 instead of yolk. The outer layer is called the trophoblastic 

 ectoderm or trophoblast ; it probably corresponds to the 

 extra-embryonic ectoderm of the chick (p. 585) and takes 

 no direct part in the formation of the embryo, which arises 

 from the inner mass (embryonic area) , in much the same way 

 as the chick is formed from the blastoderm in the case of the 

 fowl's egg. Thus later on an embryonic portion with a 

 primitive streak, and an extra-embryonic portion gradually 

 growing round beneath the trophoblast, can be recognised 

 in the embryonic area, the body of the embryo being gradu- 

 ally folded off from the blastocyst, which is then often known 

 as the umbilical vesicle and has similar relations to the embryo 

 that the yolk-sac has to the body of the chick, to which it 

 corresponds (see p. 582 and Figs. 159 and 166). 



In the lancelet alone amongst the triploblastic animals 

 described in this book does the mesoderm arise as a 

 series of enterocoelic pouches : as we have seen, it is 

 usually at first solid, and may be budded off from the 

 ectoderm and endoderm, or from the ectoderm only, 

 at the margin of the blastopore or primitive groove 

 (chick, p. 583) ; or both endoderm and mesoderm may be 

 differentiated at the same time from the lower-layer 

 cells or yolk-cells (frog, p. 202) ; or finally, it may arise 

 in all these ways. The ccelome is formed secondarily 

 by a split taking place in the mesoderm on either side 

 (Figs. 65 and 155), the split gradually extending with the 



