ORIGIN OF THE MESODERM 



65 



appears to be closely comparable with that of Reptiles and Amphib- 

 ians. Here, at the time when the mesoderm begins to make its 

 appearance, the position of the notochord is occupied by the primitive 

 streak. The mesoderm forms a loose sheet of irregularly shaped cells 

 spreading out on each side and added to from two distinct sources : 

 on its inner side by proliferation from the primitive streak and on 

 its outer side by delamination from the endoderm of the germ wall. 

 It will facilitate comprehension of the evolutionary changes which 



Fig, 40. — Semi-diagrammatic transverse sections through the embryos of various vertebrates 

 to illustrate the origin of the mesoderm. 



A, Amphioxus ; B, Petromyzon ; C, Lepidosiren ; D, Amphibian ; E, Elasmobranch. ect, ectoderm ; 

 end, endoderm ; ant, enteric cavity ; vies, mesoderm ; N, notochord ; n.r, neural rudiments. The 

 small crosses indicate regions in which active extension of the mesoderm is taking place. 



the writer believes to have taken place in the mode of development 

 of the mesoderm within the phylum Vertebrata if the main steps are 

 summarized in a diagram. In Fig. 40, A shows the primitive condi- 

 tion where the mesoderm segments are in the form of enterocoelic 

 pockets (Amphioxus). In B, with increasing amount of yolk, the 

 hollow pocket is represented by a solid block in which the cavity 

 will develop secondarily (Petromyzon). In C the condition is similar 

 but the dorsal portion of the embryonic body is more flattened out, 

 the bulk of the yolky endoderm over which it is spread having 

 VOL. II P 



