MAMMALIA. 



223 



whicli must be regarded as the allantoic stalk', and the embryo and yolk- 

 sack filled np bnt 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, 

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

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

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



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

 older than the above embr)Os 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. 1 65 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 its largest diameter, and was nearly completely covered 

 with simple villi, more develo[)ed on one side than on the other. 



In a somewhat later period, about the stage of a chick at the end of the 

 second day, the medullary folds are completely closed, the legion of the 

 brain already marked, and the cranial flexure commencing. The mesoblast 

 is divided up into numerous somites, and the mandibular and first two 

 branchial arches are indicated. The embryo is still but incompletely folded 

 off from the yolk-sack below. 



In a still older stage the cranial flexure becomes still more pronounced, 

 placing the mid-brain at the end of the long axis of the body. The body 

 also begins to be ventrally curved (fig. 165 C). 



Externally human embryos at this age are characterized by the small 

 size of the anterior end of the head. 



The flexure goes on gradually increasing, and in the third week of 

 pregnancy in embiyos of about 4 mm. the limbs make their appearance. 



Fi(i. 166. Two VIEWS OF a human embryo of between the third and fourth week. 



A. Side view. (From Kolliker; after Allen Thomson.) a. amnion; 6. umbilical 

 vesicle; c. mandibular arch; e. hyoidarch; /. commencing anterior limb; g. primitive 

 auditory vesicle ; /;. eye; i. heart. 



B. Dorsal view to shew the attachment of the dilated allantoic stalk to the 

 chorion. (From a sketch by Allen Thomson.) am. amnion; all. allantois; ys. yolk-sack. 



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 embrj-os 

 examined by him. 



