DEVELOPMENT OF THE CHICK 119 



streak elongates in a posterior direction, the 

 pellucid area elongating commensurately. In 

 this way the area becomes first oval and then 

 pear-shaped. During its elongation the streak 

 develops a primitive groove on its surface. 

 The groove is bounded laterally by the primitive 

 folds (Fig. 57) and ends anteriorly in the primi- 

 tive pit. The anterior extremity of the primitive 

 streak does not become invaded by the groove. 



The embryo itself makes its appearance in 

 front of the primitive streak, which gradually 

 diminishes in length as the embryo elongates. 

 How much of the embryo is represented in the 

 primitive streak is still open to question ; but 

 it seems certain that the view formerly held 

 that the streak contributes nothing to the 

 formation of the embryo is no longer tenable.* 



Separation of the Embryo. — The formation of 

 the embryo is first indicated on the surface of 

 the blastoderm by the appearance, during the 

 second half of the first day of incubation, of a 

 neural groove (Fig. 58) bounded laterally by the 

 longitudinal medullary folds. The folds meet 

 each other and fuse, thus converting the groove 

 into the neural tube (Fig. 59) from which arise 

 the brain and spinal cord. The union of the 

 folds is first effected in that region which will 



