MAMMALIA. !!.-) 



in. embryonic niesoblast ; in', noil-embryonic mesoblast; ilf. area vasculosa; at. sinus 

 terminalis; dd. embryonic hypoblast; /. non-embryonic hypoblast ; /.//. cavity of blas- 

 toderuiic vesicle, the greater part of which becomes the cavity of the umbilical vesicle ds. ; 

 dg. stalk of umbilical vesicle ; /. allantois ; c. embryo ; ?. space between chorion and 

 aninion containing albiiniinous fluid; vl. ventral body wall; lih. pericardia! cavity. 



of the sinus terminalis, and the posterior and smaller branch is continued 

 towards the hind part of the sinus, near which it ends. On its way it 

 receives, on its outer side, numerous branches from the venous network, 

 which connect by their anastomoses the posterior branch of the vitelline 

 vein and the sinus terminalis. 



While the above changes have been taking place the whole blasto- 

 dermic vesicle, still enclosed in the zona, has become attached 

 to the walls of the uterus. In the case of the typical uterus with 

 two tubular horns, the position of each embryo, when there are 

 several, is marked by a swelling in the walls of the uterus, prepa- 

 ratory to the changes" which take place on the formation of the 

 placenta. In the region of each swelling the zoua around the blasto- 

 dermic vesicle is closely embraced, in a ring-like fashion, by the 

 epithelium of the uterine wall. The whole vesicle assumes an oval 

 form, and it lies in the uterus with its two ends free. The embryonic 

 area is placed close to the mesometric attachment of the uterus. In 

 many cases peculiar processes or villi grow out from the ovum (fig. 

 147, 4, sz), which fit into the folds of the uterine epithelium. The 

 nature of these processes requires further elucidation, but in some 

 instances they appear to proceed from the zona (the Rabbit) and in 

 other instances from the subzoual membrane (the Dog). In any 

 case the attachment between the blastodermic vesicle and the uterine 

 wall becomes so close at the time when the body of the embryo is 

 first formed out of the embryonic area, that it is hardly possible to 

 separate them without laceration ; and at this period from the 8th 

 to the 9th day in the Rabbit it requires the greatest care to remove 

 the ovum from the uterus without injury. It will be understood of 

 course that the attachment above described is at first purely super- 

 ficial and not vascular. 



Shortly after the establishment of the circulation of the yolk-sack 

 the folds of the amnion meet and coalesce above the embryo (fig. 147, 

 3 and 4, am). After this the inner or true amnion becomes severed 

 from the outer or false amnion, though the two sometimes remain 

 connected by a narrow stalk. Between the true and false amnion is a 

 continuation of the body-cavity. The true amnion consists of a layer 

 of epiblastic epithelium and generally also of somatic mesoblast, 

 while the false amnion consists, as a rule, of epiblast only ; though it 

 is possible that in some cases (the Rabbit ?) the mesoblast may be 

 continued along its inner face. 



Before the two limbs of the amnion are completely severed, the 

 epiblast of the umbilical vesicle becomes separated from the meso- 

 blast and hypoblast of the vesicle (fig. 147, 3), and, together with the 

 false amuion (sJi), with which it is continuous, forms a complete lining 



132 



