544 MAMMALS. 



DIAGRAM XXXI. 



Development of Mammals. 



The lowest figure (after Waldeyer) shows an ovum, surrounded by a 

 porous membrane, the zona radiata, outside which are a few follicular 

 cells (f.c). The nucleus (».) or germinal vesicle, and the nucleolus or 

 germinal spot {g.s.) are also shown. 



A. B. and C. (after van Beneden) show the segmentation of the 

 ovum. Externally is an egg-envelope, between this and the ovum is a 

 clear mucous zona pellucida {z.p.). The cells of the epiblast {ef.) are 

 light, those of the hypoblast {hy.) are darkly shaded. 



In the middle of the page is a surface view of the embryonic sac. 

 The clear region in the centre {smb. a. ) is the embryonic area, with a 

 hint of the primitive streak (p.s.). The dark area round about is part 

 of the blastocyst. 



Figs I and 2 (after KoUiker) show the development of the fcetal 

 membranes: — emb. the embryo; g. the gut of the embiyo; y.s. the 

 yolk-sac; am. the amniotic fold ; ajn. «.. the amniotic cavity bounded 

 by the amnion proper; al. the allantois growing out from the gut. 

 Externally are the preliminary villi of the trophoblast. 



Fig. 3 (after Turner) shows the relations of the foetal membranes : — 

 E, the epiblast of the embryo; M, the mesoblast; H, the hypoblast 

 lining the gut ; UV, the umbilical vesicle or yolk-sac ; A C, the amniotic 

 cavity bounded by the amnion proper (am.); ALC, the cavity of the 

 allantois [al.); s.z. the subzonal membrane or outer limb of the amniotic 

 fold ; z.p, may represent the external epiblast or trophoblast with its 

 preliminary villi ; where the yolk-sac unites with the subzonal membrane 

 a yolk-sac placenta may be developed ; where the allantois unites with 

 the subzonal membrane the final placenta is formed. 



