DEVELOPMENT OF BIKDS AND MAMMALS 239 



lip of the blastopore. As the area pellucida lengthens, 

 becoming oval, pear-shaped, and then hourglass shaped, the 

 neural plate is gradually extended and the blastopore is carried 

 backwards. The neural folds become more distinct and, as in 



FIG. 122. Transverse sections of a lesser black- backed 'gull in the stage of 

 fig. 121 (1). The numbers refer to the number ,of |sections of .8/* 

 intervening between the sections drawn. 



the other cases we have examined, they limit the nervous 

 system in front transversely and extend backwards as diverging 

 folds, ending, as in the case of the frog, on each side of the 

 primitive groove. During the first day, as the neural area 



