122 



A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



streak. This layer of cells is thinnest behind and lies uniformly close to the ento- 

 derm, to which its cells are attached by numerous fine processes. The constit- 

 uent cells vary considerably in size and shape; most of them possess a number of 

 larger or smaller, partly anastomosing processes, while some seem to have a smoothly 

 rounded cell-body. One finds quite generally a fine, sharp line on that surface 

 of the mesoderm toward the ectoderm, very much like a basement membrane of 

 connective-tissue origin, the membrana prima of von Spee. There is no indication 

 anywhere of an arrangement of the mesodermic cells in two layers, as has been 

 repeatedly described in the primitive-streak region. 



The majority of the mitotic figures observed in this specimen are found near, 

 or at a short distance from, the primitive streak; by far the greater number of these 

 occur in the ectoderm and mesoderm, especially in the former; only rarely are they 



Fio. 4. Head process and completion plate. Dorsal view, but in the plane of the sections (i. e., some foreshortening. 

 Cf. fig. 3). On the left is the anterior end of the primitive streak; immediately anterior to this is the dorsal opening 

 of the archenteric canal. The four ventral openings of the canal are seen in the center of the figure; dotted lines 

 indicate that the lumen is indistinct or doubtful. The small dotted ring near the anterior end represents a very 

 doubtful cavity, and just beyond this is shown the line where the entoderm becomes a distinct layer. Marginal 

 lines as before. 



seen in the entoderm. In those cases in which the axis of the spindle can be deter- 

 mined it is found in nearly all instances parallel to the surface of the ectoderm or 

 mesoderm and at right angles to the median line. 



As mentioned above, the anterior part of the primitive groove is but the bottom 

 of a deep, median furrow in the blastoderm. This furrow becomes rapidly wider and 



