RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 867 



the main formed by a proliferation from an axial strip of the 

 epiblast along the line of the primitive streak, but in part also 

 from a simultaneous differentiation of hypoblast cells also along 

 the axial line of the primitive streak. The two parts of the 

 mesoblast so formed become subsequently indistinguishable. 

 The second part of the mesoblast to be formed is that which 

 gives rise to the lateral plates of mesoblast of the head and 

 trunk of the embryo. This part appears as two plates one on 

 each side of the middle line which arise by direct differentia- 

 tion from the hypoblast in front of the primitive streak. They 

 are continuous behind with the lateral wings of mesoblast 

 which grow out from the primitive streak, and on their inner 

 side are also at first continuous with the cells which form the 

 notochord. 



In addition to the parts of mesoblast, formed as just de- 

 scribed, the mesoblast of the vascular area is in a large measure 

 developed by a direct formation of cells round the nuclei of the 

 germinal wall. 



The mesoblast formed in connection with the primitive 

 streak gives rise in part to the mesoblast of the allantois, and 

 ventral part of the tail of the embryo (?), and in part to the 

 vascular structures found in the area pellucida. 



With reference to the formation of the mesoblast of the 

 primitive streak, our conclusions are practically in harmony 

 with those of Koller ; except that Koller is inclined to mini- 

 mise the share taken by the hypoblast in the formation of the 

 mesoblast of the primitive streak. 



Gerlach, with reference to the formation of this part of the 

 mesoblast, adopts the now generally accepted view of Kolliker, 

 according to which the whole of the mesoblast of the primitive 

 streak is derived from the epiblast. 



As to the derivation of the lateral plates of mesoblast of the 

 trunk from the hypoblast of the anterior part of the primitive 

 streak, our general result is in complete harmony with Gerlach's 

 results, although in our accounts of the details of the process we 

 differ in some not unimportant particulars. 



As to the origin of the notochord, our main result is that 

 this structure is formed as an actual thickening of the primitive 

 hypoblast of the anterior part of the area pellucida. We find 



