MAMMALIA. 



267 



regarded as the allantoic stalk 1 , and the embryo and yolk-sack filled up 

 but a very small part of the whole cavity of the vesicle. 



The embryo, which was probably not quite normal (fig. 165 -A),- was 

 very imperfectly developed ; a medullary plate was hardly indicated, and, 



etui 



FIG. 165. THREE EARLY HUMAN EMBRYOS. (Copied from His.) 



A. An early embryo described by His from the side, am, amnion ; urn. umbilical 

 vesicle ; ch. chorion, to which the embryo is attached by a stalk. 



B. Embryo described by Allen Thomson /about 12 14 days. um. umbilical 

 vesicle ; md. medullary groove. 



C. Young embryo described by His. um. umbilical vesicle. 



though the mesoblast was unsegmented, the head fold, separating the 

 embryo from the yolk-sack (um), was already indicated. The amnion (am) 

 was completely formed, and vitelline vessels had made their appearance. 



Two embryos described by Allen Thomson (No. 239) are but slightly 

 older than the above embryos of His. Both of them probably belong to the 

 first fortnight of pregnancy. In both cases the embryo was more or less 

 folded off from the yolk-sack, and in one of them the medullary groove was 

 still widely open, except in the region of the neck (fig. 165 B). The allantoic 

 stalk, if present, was not clearly made out, and the condition of the amnion 

 was also not fully studied. The smaller of the two ova was just 6 mm. in 



1 Allen Thomson informs me that he is very confident that such a form of attach- 

 ment between the hind end of the embryo and the wall of the vesicle, as that described 

 and figured by His in this embryo, did not exist in any of the younger embryos 

 examined by him. 



