no 



THE DEVELOPMENT OF THE CHICK 



beginning on each side of the head-process and primitive streak, 

 and extending laterally and posteriorly to the margin of the 

 vascular area. The lateral margins at this time extend anterior to 

 the embryonic axis, so that the anterior margin of the mesoblast 

 forms a curve with the concavity directed forward. 



c O P- 



Fig. 64. — Head of the same embryo from 

 below. X 30. Abbreviations as before. 



The mesoblast in the region in front of the primitive streak 

 is known as gastral mesoblast, and in the region of the primitive 

 streak as prostomial mesoblast; the latter is fused with the primi- 

 tive streak. However, the distinction between the gastral and 

 prostomial mesoblast is not of permanent significance, because 

 the latter is being continually converted into the former as the 

 primitive streak vmdergoes separation into ectoderm, notochord^ 

 and mesoderm. 



Confining our account now to the gastral mesoblast: a trans- 

 verse section across an embryo in which the head-fold is forming 

 shows a sheet of cells lying on each side of the notochord between 

 the ectoderm and entoderm. It is several cells deep near the 

 notochord, and thins gradually peripherally (cf. Fig. 56). The 

 thicker portion next the notochord is distinguished as the paraxial 

 mesoblast (vertebral plate) from the more peripheral portion or 

 lateral plate. The mesoblast is sparser, the cells more scattered, 



