264 DEVELOPMENT OF THE GUINEA-PIG. 



The previously cylindrical egg next assumes a spherical form, and the 

 mesoblast arises in connection with the primitive streak in the manner 

 already described. A splanchnic layer of mesoblast attaches itself to the 

 inner side of the outer hypoblastic wall of the egg, a somatic layer to the 

 epiblast of the inner vesicle, and a mass of mesoblast grows out into the 

 cavity of the larger vesicle forming the commencement of the allantois. 

 The general structure of the ovum at this stage is represented on fig. 162, 

 copied from Schafer ; and the condition of the whole ovum will best be 

 understood by a description of this figure. 



It is seen to consist of two vesicles, (i) an outer larger one (h] the 

 original egg-cylinder united to the mesometric wall of the uterus by a vas- 

 cular connection at m'm' t and (2) an inner smaller one (ev) the originally 

 solid body at the free end of the egg-cylinder. The outer vesicle is formed 

 of (i) an external lining of columnar hypoblast (h} which is either pierced or 

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

 inner layer of splanchnic mesoblast (;") 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 (all], so far with- 

 out hypoblast 1 . 



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 oittside 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 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. 



1 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. 



