268 



HUMAN OVUM. 



its largest diameter, and was nearly completely covered with simple villi, 

 more developed 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 region 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 characterised 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 embryos of about 4 mm. the limbs make their appearance. 

 The embryo at this stage (fig. 166), which is about equivalent to that of a 



FIG. 166. Two VIEWS OF A HUMAN EMBRYO OF BETWEEN THE THIRD AND 



FOURTH WEEK. 



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

 vesicle ; c, mandibular arch ; e. hyoid arch ; f. commencing anterior limb ; g. primitive 

 auditory vesicle ; h. 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. 



chick on the fourth day, resembles in almost every respect the normal 

 embryos of the Amniota. The cranial flexure is as pronounced as usual, 

 and the cerebral region has now fully the normal size. The whole body 

 soon becomes flexed ventrally, and also somewhat spirally. The yolk- 

 sack (b} forms a small spherical appendage with a long wide stalk, and the 

 embryo (B) is attached by an allantoic stalk with a slight swelling (all], 

 probably indicating the presence of a small hypoblastic diverticulum, to the 

 inner face of the chorion. 



A remarkable exception to the embryos generally observed is afforded 

 by an embryo which has been described by Krause (No. 235). In this 



