28 Vertebrate Embryology 



cells, which is a characteristic feature of all 

 transverse sections of embryos (Figs. lo, 12, 

 13, and 15). 



According to some workers the notochord 

 is formed by a condensation and differentiation 

 of mesoblast cells along the mid-dorsal region 

 of the embryo. Since the mesoblast and ento- 

 blast are, in their origin, so closely associated, 

 the exact method of formation of the noto- 

 chord, whether from the one layer or the other, 

 is not a matter of very great importance ; but 

 the majority of workers probably support the 

 former view, — that is, that it is formed by a dif- 

 ferentiation of a part of the entoblastic layer. 



Like the formation of the archenteron, the 

 origin of the middle germ-layer, or mesoblast, 

 has been so variously described by different 

 authors that it is quite a difficult matter to 

 decide which is the most probable view. Al- 

 though so difficult of determination in the 

 case of the frog, the origin of the mesoblast 

 in some other animals is easily made out. 



Marshall states that it "arises in the frog as 

 two lateral sheets of cells, split off from the 

 outer surface of the hypoblast and yolk-cells." 



Morgan says " . . . the cells that are to form 

 the mesodermal layer are present at the time when the 



