A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 121 



easily determined, and our reason for assigning the posterior limit is that at this 

 latter point there is not only a conspicuous connection of mesoderm and ectoderm, 

 but also a rather sudden separation of the entoderm from the cell-mass just above. 

 This cleft between the entoderm and the primitive streak is present throughout its 

 caudal half, while anterior to this the entoderm lies very close to the newly formed 

 mesodermic elements. 



The primitive groove is best defined at its posterior end, where it appears as 

 a sharply outlined groove between the more gentle slopes of the ectoderm on either 

 side. Farther forward, where the high ectodermic folds are found, this fine median 

 furrow can not be distinguished, being simply the bottom of a deep, narrow trough. 

 In its most caudal part the primitive groove possesses a narrow, flat floor bounded 

 by perpendicular walls of distinctly greater extent. The breadth of the floor may 

 be estimated at about 0.01 to 0.015 mm. This dimension is accentuated in the 

 photographs on account of the obliquity of the sections. Followed forward, the 

 groove varies in shape and width, its floor soon disappears, and it is finally lost in 

 the general slope of the embryonic ectoderm. In the primitive groove the outlines 

 of the ectodermic elements, free surface, and cell boundaries are more distinct and 

 the arrangement of the nuclei is rather more regular than elsewhere. 



In those places where there is a well-defined floor one can sometimes see, in 

 one side of the floor, a deep, distinct secondary groove, usually on the right. A few 

 sections show a definite lipping of the primitive groove, i. e., the lateral wall; this 

 being observed only on the left side, bulges into the groove, forming a small recess 

 between the floor and the wall. These peculiar conditions occur only near the 

 posterior end of the primitive groove. The ectoderm of the groove presents 2 to 

 3 or 4 layers of nuclei and is thickest in front. The floor is often composed of but 

 a single layer of low cells, remarkably thin in places but always intact. 



From the walls of the primitive groove, much less conspicuously from its floor, 

 where this is well marked, the ectoderm is continued directly into the mesoderm 

 lying laterally, while the floor of the groove, often much thinner than the side-walls, 

 is widely separated from the entoderm, in which space occasional free cells may be 

 seen. There are in a few places interruptions in the transition from ectoderm to 

 mesoderm, probably artefacts due to the loose character of the latter layer. The 

 width of the primitive streak, i. e., of the zone of proliferation of mesoderm, is about 

 0.05 mm. Towards its anterior end this zone gradually becomes more massive, the 

 connection between ectoderm and mesoderm more extensive, and the entire median 

 line between ectoderm and entoderm becomes filled with closely packed mesodermic 

 cells. At the very anterior end of the streak, just behind the primitive node, the 

 lower germ-laj'er is again widely separated from the cells dorsal to it. Everywhere, 

 however, in the primitive streak can the entoderm be seen as a distinct, definite 

 cell-layer. 



The mesoderm on either side of the middle line forms a well-defined, loosely 

 cellular stratum of slightly varying thickness. It is continuous laterally and poste- 

 riorly with the outer layers of the yolk-sac and amnion, anteriorly with the mesoderm 

 on either side of the head process, and mesially with the ectoderm of the primitive 



