HUMAN EMBRYO IN THE SIXTH STAGE. 



137 



satisfactory is the embryo described by Dandy, a median section of which is given 

 in figure 25. The embryo in "E" measured (?) 2.1 mm.; in "SR," 2.2 mm. 

 (Fig. 82). It will be noticed at once that .the condition is very similar to that 

 shown in figure 80, but the embryo is somewhat more advanced. The most 

 important changes in the embryo at this stage are its general growth, so that it 

 rises above the yolk and has both projecting head and projecting tail. The 

 medullary groove is very deep and extends the entire length of the embryo. Toward 

 its caudal end it probably has an open neurenteric canal. The dorsal outline of 



Cho 



Md. 



Am. 



Yk. 



FIG. 82. HUMAN EMBRYO WITH OPEN MEDULLARY GROOVE. 

 Am, Amnion. b.s, Body-stalk. Cho, Chorion. Md, Medullary folds. Yk, Yolk-sac. (After W. His.) 



the embryo is somewhat concave. On the under side of the projecting head, 

 between it and the anterior limit of the yolk-sac, the anlage o'f the heart has 

 appeared, and its cavity may be supposed to be in connection with the blood- 

 vessels of the yolk-sac. The development of segments has begun; Dandy's embryo 

 had seven. From the under side of the projecting tail end springs the body-stalk, 

 to the distal end of which the chorion is attached. The chorion is completely 

 covered by short branching villi. The yolk-sac has still a very broad connection 

 with the embryo, and contains blood-vessels throughout its entire extent. The 

 space between it and the chorion, the extra-embryonic coelom, is very large. 



Human Embryo in the Sixth Stage with Medullary Canal. 



This stage does not include the whole period from the beginning to the com- 

 pletion of the closure of the medullary groove to form the medullary canal, but 

 only the first part of this period. The best-known specimen of this stage was 

 described by Kollmann. It measured 2.2 mm. in length and had the medullary 

 groove open through the anterior two thirds of its length, but closed along the 

 caudal third. The embryo had thirteen segments (Fig. 83). The yolk-sac was 

 attached to the embryo for a distance of 1.5 mm., leaving the head to project 



