388 THE PHYSIOLOGY OF REPRODUCTION 



much reduced, and was " presumably enveloped by the allantois." 

 It had previously been shown by Turner that the yolk-sac disap- 

 peared at an early period. 



RODENTIA. In Rodents the conditions are entirely different. 

 The mesoblast never extends, in the rabbit, rat, or mouse, 

 completely round the ovum, and the yolk-sac hypoblast remains 

 long in contact with the trophoblast, and carries on the nutrition 

 of the embryo till the tardily formed allantoic placenta is de- 

 veloped. Regarding the partial extension of the mesoblast, 

 Minot l says : " That it represents a modified condition is 



Ent 

 EC 



FIG. 83. Blastodermic vesicle of the rabbit. (Minot.) 



co?, coelom ; Cho, ch^orion (diplo-trophoblast) ; Yk, yolk-sac; mes, raesoderm ; 

 v t, vena terminalis ; Ent, entoderm ; EC, ectoderm. 



evident, since in all non-mammalian Vertebrates both mesoderm 

 and coelom extend completely round the yolk. Hence the com- 

 plete separation of the yolk-sac in Man and the sheep is nearer 

 the ancestral type than the relations of the extra-embryonic 

 germ-layers to one another in the rabbit and opossum." 



In the rabbit, the mesoblast begins to spread out from the 

 embryonic region about the end of the first week of gestation, 

 and it gradually reaches half-way round the circumference of the 

 blastocyst. It splits into two layers over its whole extent, and 

 it is limited below by the sinus terminalis (Fig. 83). The lower 

 half of the yolk-sac is non- vascular, and its wall of hypoblast is 

 closely invested by trophoblast. Later the yolk-sac begins to 



1 Minot, Human Embryology, Boston, 1892. 



