310 STUDY OF THE BLASTODERMIC VESICLE. 



together with their smaller darker nuclei, make them readily recognizable. The 

 granules in the entodermic nuclei are smaller and more numerous than in the 

 overlying ectodermal nuclei. 



During the next few hours further changes ensue. At the apex of the pear- 

 shaped mesodermic area there appears a small spot, which is known as Hensen's 

 knot. At first Hensen's knot consists of a little thickening accompanied by a 

 union of the cells of the middle layer with those of the overlying ectoderm. Next 

 occurs the development of the primitive streak, which runs from Hensen's knot 

 backward toward the apex and embryonic shield, and very soon thereafter along 

 the line of the primitive streak there develops the external and shallow primitive 

 groove. At Hensen's knot the three layers now are found to be intimately 

 united, so that, though they may everywhere else, when fresh, be separated from 

 one another, the germ-layers at this point cannot be separated, except by tearing. 

 Finally, in the next stage there grows out in front of Hensen's knot the so-called 

 head process, an axial band of cells in which the notochordal canal develops. 



Ent. 



FIG. 176. RABBIT EMBRYO OF SEVEN DAYS. TRANSVERSE SERIES 12, SECTION 216, THROUGH THE 



ANTERIOR PORTION OF THE EMBRYONIC SHIELD. 



EC, Ectoderm. Ent, Entoderm. X 3 diams. 



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

 the two sections represented in figures 176 and 177, 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 1 76 shows 

 the middle portion of a section. It consists merely of the outer, thicker, ecto- 

 dermar layer, EC, and the very thin entodermal 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 ectodermal cells is lightly 

 stained, and granular in appearance. The boundaries between adjacent cells are 

 indicated by delicate lines, which extend through the entire thickness of the 

 ectoderm, which is now but a single layer of cells. The original outer (Rauber's) 

 layer has disappeared. The entoderm is very thin, but is thickened a little 



