THE DEVELOPMENT OF VERTEBRATA. 



253 



line becomes differentiated into a neural plate. Along the 

 sides of this the neural folds arise exactly as in the frog. 

 They extend behind alongside the anterior part of the 

 primitive streak. 



The notocJwrd is formed just as in 

 the frog. 



The mesoblast of either side, soon 

 after its formation splits (as in the 

 frog) to form the right and left 

 portions of the co3lom : the union 

 of these right and left portions is 

 necessarily delayed for a long time 

 by the presence of the huge mass of 

 yolk. As the area opaca gradually 

 spreads over the yolk, the differentia- 

 tion of the three germinal layers 

 spreads with it, and the splitting of 

 the mesoblast follows close behind. 



The splitting is soon followed by 

 the division into vertebral and lateral 

 plates, and the former soon begin to divide transversely 

 into somites (proto vertebra), beginning in the region just 



ig. 129. SURFACE VIEW OF 

 BLASTODERM, SECOND DAY 

 OF INCUBATION. 



(After Balfour.) 



3P/ti/AL COM 



vTO**^7VC 



Fig 130. TRANSVERSE SECTION OF BLASTODHUM OF CHICK, 

 SECOND DAY OF INCUBATION. 



(After Balfour.) 



behind the head and gradually increasing by the cutting- 

 off of additional ones behind. 



These changes, up to the formation of the first two or 

 three pairs of somites, represent the progress of the first 

 twenty-four hours of incubation. By this time, as already 



