132 



THE HUMAN EMBRYO. 



cave. On the under side of the projecting head, between it and the anterior 

 limit of the yolk-sac, the anlage of the heart has appeared, and its cavity may be 

 supposed to be in connection with the blood-vessels of the yolk-sac. The devel- 

 opment of segments has begun ; how many were present in either of these em- 

 bryos is uncertain. 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. 



Fig. 68. — Human Embryo of from Thirteen to Fourteen Days. 

 Am, Amnion. S.y, Seventh segment. Md, Medullary groove. A?, Heart. Yk.s, Yolk-sac. .41, Body-stalk. 



— (After J. Kallmann.) 



Human Embryo in the Sixth Stage with Medullary Canal. 



This stage, if we define it to include the whole period from the beginning to 

 the completion of the closure of the medullary groove to form the medullary 

 canal, covers a considerable epoch of development. 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. 68). 

 The yolk-sac was attached to the embryo for a distance of 1.5 mm., leaving the 

 head to project 0.58 mm. and the tail to project 0.3 mm. The head is already 

 somewhat enlarged and slightly bent over toward the ventral side. It forms at 

 least one-third of the whole embryo. The dorsal outline of the embryo is con- 

 cave in the region where the segments have developed. The caudal end is 

 slightly curved over and is connected on its under side with the body-stalk, Al, 

 by which the embryo is attached to the chorion. Between the yolk-sac, Yk. s, 

 and the head, the heart, Ht, is prominent. By analogy with other vertebrates 

 we assume that the heart tube, when it first appears in man, is straight and 



