PRIMITIVE STREAK TO SOMITE FORMATION 35 



entoderm are continuous with each other without any demarca- 

 tion. The mesoderm arises from the primitive streak where 

 ectoderm and entoderm merge and grows laterad on both sides of 

 the primitive streak extending into the space between ectoderm 

 and entoderm. The mass of cells in the floor of the primitive 

 groove is to be regarded as constituting an undifferentiated area 

 from which new cells are being proliferated rapidly and are 

 emigrating to become components of one or another of the germ 

 layers. 



To those who have studied the embryology of more primitive 

 vertebrates, particularly the Amphibia, the fact that the lips 

 of the blastopore constitute centers of growth from which cells 

 are pushed forth to take part in the formation of the differenti- 

 ated germ layers will already be familiar. The fact that the 

 blastopore of the chick has suffered a change in position due to 

 concrescence, and has in the same process become closed by 

 fusion of its lips must not be allowed to obscure its homologies. 

 In attempting to bring the relationships of the germ layers in 

 the chick into line with the relationships of the germ layers in 

 embryos having less yolk, it will be of great assistance to picture 

 a chick lifted off the yolk and the lateral margins of the blasto- 

 derm pulled together ventrally; or, the method of comparison 

 may be reversed if one imagines the embryo of a form having 

 less yolk, such as an amphibian, to be split open along the mid- 

 ventral line and spread out on the surface of a sphere as a chick 

 lies on the yolk. 



In Figure 13, D y a small region at the primitive streak has 

 been drawn at higher magnification to show the characteristic 

 cellular structure of the undifferentiated region in the floor of 

 the primitive groove and of the various layers merging at this 

 place. The cells of the ectoderm are much more closely packed 

 together and more sharply delimited than those of the other 

 germ layers. Where the ectoderm is thickened in the primitive 

 ridge region, it is several cell layers thick (stratified). (Fig. 13, 

 D.) In regions lateral to the primitive ridge it gradually be- 

 comes thinner until it consists of but a single cell layer (Fig. 13, 

 E). The rapid extension that the mesoderm is at this time 

 undergoing is indicated by the loose arrangement and sprawling 

 appearance of its cells. Their irregular cytoplasmic processes 

 make them look much like amcebae fixed during locomotion. 



