138 



THE HUMAN EMBRYO. 



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 concave 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 occupies a longitudinal median 



, 



-Am 



Ht 



FIG. 83. HUMAN EMBRYO OF FROM THIRTEEN TO FOURTEEN DAYS. 



Am, Amnion. 5.7, Seventh segment. Md, Medullary groove. Ht, Heart. Yk.s, Yolk-sac. Al, Body-stalk 



(After J. Kollmann.) 



position. In this embryo it has already become a relatively large organ and the 

 tube itself is strongly bent. No anlage of the eye or ear was distinguished. The 

 amnion was a thin, transparent membrane enveloping the embryo quite closely. 

 The closeness of the amnion to the embryo was probably accidental (compare Figs. 

 84 and 85). The chorion was covered externally by branching villi; its diameter, 

 including the villi, was 18 mm. 



Another embryo, the position of which in the series of known stages has long 

 been a matter of dispute, I feel, after renewed study, must be assigned to a place 

 very close to Kollmann's embryo just described. The specimen in question was 

 figured by Coste in his . monumental "Atlas of Embryology."* The embryo was 

 enclosed in a villous chorion (Fig. 84) and was provided with a large vitelline sac, 



* The greatest difficulty comes from Coste's statement as to the magnification of his drawings, according to 

 which the embryo must have been about 4.4 mm. long, or nearly double the length which we now know to be 

 normal for embryos in the stage in which this one seems, to be. Other difficulties arise because Coste has given 

 no further description of this embryo than that which appears in the explanation of his plate. Neither that ex- 

 planation nor the figures themselves afford any information concerning the dorsal side of the embryo or as to 

 whether it had a partially open medullary groove or not. Coste's figures indicate that thirteen or fourteen seg- 

 ments were visible externally. The shape of the head, the size and curvature of the heart, the form of the tail, 

 and the concavity of the dorsal outline in the segmented region of the embryo all indicate an extremely close resem- 

 blance to Kollmann's embryo.. As Coste's figures were all made from fresh specimens freehand, we shall prob- 

 ably commit no error if we assume that the magnification was not correctly given. By making this assumption I 

 think the difficulties as to placing Coste's embryo vanish. 



Coste's private collection was said to be at the College of France, but upon search this specimen could not 

 be found, so that attempts to ascertain its actual length were without result. 



