110 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. 
ap. 
Fic. 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 undergoes 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 (ef. 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, 
