THE EARLY DEVELOPMENT OF THE CHICK 253 



the primitive groove terminates in a small depression, the 

 primitive pit, near the anterior end of the primitive streak, where 

 a slight thickening, known as the primitive knot or Hensen's 

 node, is visible (Fig. 97) . In front of this another larger though 

 less definite thickening, known as the head process, soon may 

 be made out. Posteriorly the primitive streak is somewhat 

 transversely extended, just within the border of the area pel- 

 lucida, forming a more or less well marked crescentic area, 

 through which the primitive groove may be continued, giving 

 this a bifurcated appearance here. Between this broadened 

 hinder end of the primitive streak and the margin of the pellu- 

 cid area, a wide thickened region can sometimes be discerned; 

 this is known as the primitive plate. 



Many important structural details of the primitive streak 

 region may be determined through the study of sections. Fig- 

 ures 94, 101 illustrate sections through representative regions 

 of an early primitive streak. The ectoderm is broadly thick- 

 ened as the rudiment of the medullary plate, and along the 

 mid-line is the primitive streak, soon marked by the rapid pro- 

 liferation of ectoderm cells, which are thrown off in the space 

 between ectoderm and endoderm. These cells are the rudi- 

 ment of the mesoderm. The endoderm forms a uniformly thin 

 sheet of cells across the entire blastoderm. 



A little later (Fig. 94, B) the primitive groove is indicated, 

 bordered by a pair of primitive folds. Proliferation of mesoderm 

 cells from the inner surface of the ectoderm is now very rapid, 

 and these cells soon migrate distally throughout a large part of 

 the space between the two primary germ layers. Near the mid- 

 line they become intimately related with the endoderm, often 

 giving the misleading appearance of having been derived 

 directly from that layer (Fig. 101). Soon the cells along the 

 middle of the primitive streak have multiplied so extensively 

 that they form a dense mass, in which clearly marked limits of 

 the germ layers cannot be made out. More laterally, however, 

 the layers are completely distinct. The ectoderm extends out 

 over the germ wall and yolk; the endoderm, now more than one 

 cell thick, reaches only to the germ wall, with which it fuses * 



