MAMMALIA. 235 



surface of the subzonal membrane and united with it to form the chorion. The cavity 

 of the allantois is aborted. This fig. is a diagram of an early human ovum. 



d. zona radiata ; d '. processes of zona; sh. subzonal membrane; ch. chorion; ch.z. 

 chorionic villi; am. amnion ; ks. head-fold of amnion ; ss. tail-fold of amnion; a. 

 epiblast of embryo; a', epiblast of non-embryonic part of the blastodermic vesicle; 

 m. embryonic mesoblast; tri . non-embryonic mesoblast; df. area vasculosa; st. sinus 

 terminalis; dd. embryonic hypoblast; i. non-embryonic hypoblast; kh. cavity of 

 blastodermic vesicle, the greater part of which becomes the cavity of the umbilical 

 vesicle ds.\ dg. stalk of umbilical vesicle; al. allantois; e. embryo; r. space between 

 chorion and amnion containing albuminous fluid; vl. ventral body wall; hh. peri- 

 cardial cavity. 



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 

 blastodermic 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, preparatory to the changes which take place on 

 the formation of the placenta. In the region of each swelling 

 the zona around the blastodermic 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, 

 ss), 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 subzonal membrane 

 (the Dog). In any case the attachment between the blasto- 

 dermic 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 with- 

 out laceration ; and at this period from the 8th to the pth 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 superficial 

 and not vascular. 



Shortly after the establishment of the circulation of the yolk- 



