114 



THE HUMAN EMBRYO. 



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

 and its appendages are the same as in the previous stage (compare Fig. 66). 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 varied 

 in length, but measured about 2.2 mm. The medullary ridges are very charac- 

 teristic, rising high above the yolk-sac and enclosing a deep medullary groove 

 between them. Of this stage our knowledge is very imperfect. 



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

 or wholly closed and the heart is clearly differentiated. The embryo measures 



FIG. 52. HUMAN EMBRYO AT THE BEGINNING OF THE THIRD WEEK. 

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



from 2.2 to 2.5 mm. in length. The head projects 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. 68). The auditory 

 invagination is probably not yet formed. There are no gill clefts showing ex- 

 ternally. 



Seventh Stage. One Gill Cleft Showing Externally: Not known by obser- 

 vation. 



Eighth Stage. Two Gill Clefts Showing Externally: Several embryos in this 



