120 THE HUMAN EMBRYO. 



covering the embryo, which measures only i.o to 1.5 mm., and from the ventral 

 surface of the embryo arises the yolk-sac, which is of rounded form and about 

 equal in diameter to the length of the embryo. 



Fifth Stage. The Medullary Groove: The general relations of the embryo and 

 its appendages are the same as in the previous stage (compare Figs. 82 and 25). 

 In the cases recorded the chorionic vesicle varied greatly in size. It bore villi 

 over its entire surface, and the villi were considerably branched. The embryos of 

 this stage vary in length, but measure about 2.0 mm. The medullary ridges are 



FIG. 67. HUMAN EMBRYO AT THE BEGINNING OF THE THIRD WEEK. EIGHTH STAGE. 

 All, Allantois. Am, Amnion. -br, Branchial region. H, Head. Hr, Heart. Yk, Yolk-sac. 



very characteristic, rising high above the yolk-sac and enclosing a deep medullary 

 groove between them. During this stage the formation of the segments is progress- 

 ing. Thus one of the embryos described had seven segments. 



Sixth Stage. Medullary Tube: In this stage the medullary groove is partly 

 closed and the heart is clearly differentiated. It must be remembered that the 

 closure of the medullary groove progresses slowly and is not completed until the 

 ninth or tenth stage. The embryo measures from 2.2 to 2.5 mm. in length. 

 The head projects well in front of the yolk. The primitive segments are partly 

 developed. In one case seven, in another thirteen, were found to have been 

 formed. The caudal end of the embryo also projects beyond the yolk, but less 

 than does the head (compare Fig. 83). The auditory imagination is probably not 

 yet formed. There are no gill-clefts showing externally. 



