DEVELOPMENT OF BIKDS AND MAMMALS 287 



ectoderm is the primitive streak. The streak is at once further 

 defined by a groove which runs also longitudinally, and this 

 is called the primitive groove. It is produced by an infolding 



Neural fold 



Head fold 



*%i 'ffig&y^* * 



Amnion. 

 Somites* 



Vit.v. - ^ Vitettine a. 





Forebrain 

 Midbrain 

 . And. ves. 

 Amnion* 



Hindbrain 



I 



1st gill Heart Somite Primitive groove 



FIG. 121. A series of four diagrams to illustrate the condition of the embryo 

 (1) near the end of the first day of incubation, (2) beginning of second 

 day, (3) middle of second day, and (4) end of second day. The primitive 

 streak in each is connected by a dotted line. 



of the ectoderm. At its forward end the primitive groove 

 forms a pit, and in aquatic and some other birds this pit is still 

 more distinctly infolded to form an opening into the mass of 

 cells which unites the ectoderm with the endoderm. This pore 

 is also developed in the blastoderm of reptiles, and becomes an 

 opening into the enteron. In the case of the birds mentioned, 



