. 

 172 STUDY OF THE SEGMENTATION OF THE OVUM. 



sen's knot; C, through the primitive groove. The three primitive germ-layers are 

 easily recognized in each section. They are somewhat separated from one another, 

 but in life probably lay close together. The upper layer or ectoderm is the thick- 

 est and consists of low columnar cells; it is characteristic of the embryonic shield, 

 and at the edge (not included in the figure) of the shield changes to a thin sheet 

 of cells, which forms the outer layer of the rest of the blastodermic vesicle. The 

 lowest layer, entoderm, is a thin sheet only one cell thick. The middle layer, 

 mesoderm, is more irregular, and has begun to thicken, being in places two or 

 even three cells thick. In the median line the mesoderm enters into special rela- 

 tions with the other layers. In the middle section, B, which passes through Hen- 

 sen's knot, it forms a considerable axial thickening, which fuses with the entoderm 

 below and ectoderm above, and builds with the latter a dome-like projection. 

 The axial thickening extends forward from Hensen's knot, constituting the so-called 

 head-process, and in this region, A, the mesoderm is united only with the entoderm. 





- Ent. 



FIG. 127. RABBIT EMBRYO OF SEVEN DAYS. TRANSVERSE SERIES 12, SECTION 216, THROUGH THE ANTERIOR 



PORTION OF THE EMBRYONIC SHIELD. 

 EC, Ectoderm. Ent, Entoderm. X 350 diams. 



Behind Hensen's knot the thickening extends backward, making the primitive 

 streak, C, which is characterized by the union of the mesoderm with the ectoderm 

 only, and by the primitive groove, a shallow median longitudinal depression of the 

 ectoderm. 



The structure of the embryonic shield at seven days is further illustrated by 

 figures 127 and 128, the former passing across the anterior portion of the shield, 

 where it is two-layered, and the latter across the posterior portion, in which the 

 middle layer has appeared. Figure 176 shows the middle portion of a section. It 

 consists merely of the outer, thicker, ectodermal layer, EC, and the very thin ento- 

 dermal layer, Ent. Both surfaces of the ectoderm are quite sharply defined. The 

 nuclei are rather large and show several large, deeply stained nucleoli in each. 

 The outline of the nucleus is sharp, and, in addition to the larger granules, there 

 are many smaller ones less deeply stained scattered through the nucleus. The 

 nuclei vary considerably in size, shape, and position. The protoplasm of the ecto- 

 dermal cells is lightly stained, and granular in appearance. The boundaries between 

 adjacent cells are indicated by delicate lines, which extend through the entire thick- 

 ness of the ectoderm, which is now but a single layer of cells. The original outer 



