FORMATION OF THE PRIMITIVE STREAK 31 



somewhat later stage (Fig. 10, C) shows cells extending to 

 either side of the undifferentiated cell mass, between the ecto- 

 derm and the entoderm. These cells are the primordium of 

 the third of the germ layers, the mesoderm. The outgrowth of 

 the mesoderm and the median depression in the primitive streak 

 appear synchronously. This median depression in the primi- 

 tive streak is the primitive groove. It is not unlikely that the 

 formation of the primitive groove is due to cell rearrangement 



lips of blastopore 



A 



Tp <& ^^ 



''V^vpl^ 



yolk ^""^ ^^ ent 



primitive gut 



B 



primitive gut 



primitive groove 



primitive gut 



FIG. io. Diagrams showing schematically the relations of the germ layers 

 during the formation of the primitive streak by concrescence of the margins of 

 the blastopore. A, hypothetical section of blastoderm at the stage represented 

 in Fig. 9, B. The plane of the section is indicated by the line b-b Fig. 9, B. 

 B, hypothetical section of blastoderm at the stage represented in Fig. 9, D. 

 The plane of the section is indicated by the line d-d, Fig. 9, D. C, schematic 

 transverse section through the primitive streak at the stage represented in 

 Fig. 8. 



in this region entailed by the rapid outgrowth of the cells con- 

 stituting the mesoderm. (See arrows in Figure io, C.) 



With the formation of the mesoderm the chick has estab- 

 lished the three germ layers characteristic of all vertebrate 

 embryos. The importance of these layers lies in the uniformity 

 of their origin and history. From them the development of all 

 the organ systems may be traced. The ectoderm gives rise to 



