120 



A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



nuclei. Farther back and laterally the ectoderm is frankly one-layered, with low 

 columnar or even cuboidal cells. Throughout most of the embryo, however, the 

 arrangement of its nuclei in several layers, the character of its lower surface, and 

 its often irregularly varying thickness make impossible any definite statements as 

 to its real structure. Cell boundaries are not commonly visible. 



P406 P395- 



iD 42S (D 406 tj P6D40I (D 3Sd) P3flO P373. 



FIG. 3. Median sagittal section of embryo, yolk-sac, body-stalk, allantois, and adjacent chorion, slightly schematized, 

 about X 40. The primitive streak and head process are represented as if both lay in the same plane. The chorionie 

 villi are quite diagrammatic, their connections with each other not being indicated. Two "funnels" in the In.dy- 

 stalk, further details in text. Marginal lines as in fig. 2. 



The primitive streak (figs. 1, 2, and 3; plate 2, fig. 1; plate 3, fig. 1) is very long 

 in this embryo, making up about one-third of the axis of the blastoderm, the center 

 of the latter being just in front of the anterior end of the streak. Its length, meas- 

 ured from the anterior limit of the cloaca!' membrane to the dorsal opening of the 

 archenteric canal, is about 0.65 mm. In position it is not exactly central, but is 

 displaced a trifle to the right. The primitive groove is well defined throughout, 

 tht' continuation, as noted above, of its anterior end passes slightly to the right of 

 the opening of the archenteric canal and the beginning of the head process. This 

 condition, in which the primitive streak and its head process do not lie in the same 

 sagittal plane, is not uncommon in a variety of forms, and apparently the head proc- 

 ess is usually on the left, as in this case. The anterior limit of the streak is of course 



