128 F. M. BALFOUR AND F. DEIGHTON. 



a simultaneous differentiation of hypoblast cells also along the 

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

 blast 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 differentiation 

 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 described, 

 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 struc- 

 tures found in the area pellucida. 



With reference to the formation of the mesoblast of the primi- 

 tive streak, our conclusions are practically in harmony with those 

 of Koller ; except that Roller is inclined to minimise 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 

 that it unites posteriorly with a forward growth of the axial 

 tissue of the primitive streak, while it is laterally continuous, at 

 first, both with the mesoblast of the lateral plates and with the 

 hypoblast. At a later period its connection with the mesoblast 

 is severed, while the hypoblast becomes differentiated as a con- 

 tinuous layer below it. 



As to the hypoblastic origin of the notochord, we are again in 

 complete accord with Gerlach; but we differ from him in admitting 

 that the notochord is continuous posteriorly with the axial tissue 

 of the primitive streak, and also at first continuous with the 

 lateral plates of mesoblast. 



