220 



DEVELOPMENT OF THE GUINEA-PIG. 



invaginated at the area of vascular connection with the uterus, and (2) of 

 an inner layer of splanchnic mesoblast (»i") which covers without a break 

 the vascular uterine growth. At the upper pole of the ovum is placed 

 the smaller epiblastic vesicle, and where the two vesicles come together is 

 situated the embryonic area with the primitive streak (/"), and the 

 medullary plate seen in longitudinal section. The thinner wall of the 

 inner vesicle is formed of epiblast and somatic mesoblast, and covers over 

 the dorsal face of the embryo just like the amnion. It is in fact usually 

 spoken of as the amnion. The large cavity of the outer vesicle is continuous 

 with the body cavity, and into it projects the solid mesoblastic allantois 

 (a//), so far without hypoblast'. 



The outer vesicle corresponds exactly with the yolk-sack, and its meso- 

 blastic layer receives the ordinary vascular supply. 



The embryo becomes folded off from the yolk-sack in the usual way, but 

 comes to lie not outside it as in the ordinary form, but in its interior, 

 and is connected with it by an umbilical stalk. The yolk-sack forms 

 the substitute for part of the subzonal membrane of other Mammalia. 

 The so-called amnion appears to me from its development and position 

 rather to correspond with the non-embryonic part of the epiblastic wall 

 (true subzonal membrane) of the blastodermic vesicle of the ordinary 

 mammalian forms than with the true amnion ; and a true amnion would 

 seem not to be developed. 



The allantois meets the yolk-sack on about the seventeenth day at the 

 region of its vascular connection with the uterine wall, and gives rise to 



the placenta. A diagrammatic 



g^^,^a=g,,;g sgs;s;;^gMi _^^m representation of the structure 



of the embryo at this stage is 

 given in fig. 163. 



The peculiar inversion of the 

 layers in the Guinea-pig has 

 naturally excited the curiosity 

 of embryologists, but as yet no 

 satisfactory explanation has been 

 offered of it. 



At the time when the ovum 

 first becomes fixed it will be re- 

 membered that it resembles the 

 early blastodermic vesicle of the 

 Rabbit, and it is natui-al to sup- 

 pose that the apparently hypo- 

 blastic mass attached to the 

 inner wall of the vesicle becomes 

 the solid body at the end of the 

 egg-cylinder. This ap{)ears to be 

 Bischoff's view, but as shewn 

 above, the solid mass is really 

 the epiblast ! Is it conceivable 

 that the hypoblast in one species 



y^-- 



all- 



FlG. 163. DiAGKAMMATIC LONGITUDINAL SEC- 

 TION OF AN OVUM OF A GuiNEAPIG AND THE 

 ADJACENT UTEKINE WALLS AT AN ADVANCED STAGE 



OF PEEGNANCY. (After Bischoff.) 



yU. inverted yolk- sack (umbilical vesicle) 

 formed of an external hypoblastic layer (shaded) 

 and an internal vascular layer (black). At the 

 end of this layer is placed the sinus terminalis ; 

 all. allantois ; "pl. placenta. 



The external shaded parts are the uterine 

 walls. 



J Hensen states that the hypoblast never grows into the allantois ; while Bischoff, 

 though not very precise on the point, implies that it does ; he states however that it 

 soon disappears. 



