28 Vertebrate Embryology 



cells, which is a characteristic feature of all 

 transverse sections of embryos (Figs. 10, 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 



