THE ONTOGENESIS OF VERTEBRATES 



67 



recognized in the case of the selachians. The amphibians show 

 considerable modification, and these are the last in the ascend- 

 ing scale in which the diverticula are provided from the first 



a 



FIG. 15. Four cross-sections of vertebrate embryos showing develop- 

 ment of the mesoderm. 



(a), Amphioxus [after HATSCHEK]; (&) Triton (a salamander) [after HERT- 



WIG]; (c) bird, diagrammatic; (d) mole [after HEAPE]. 



k, ectoderm; n, endoderm; m, mesoderm; mt, parietal layer of the mesoderm; 

 mp, visceral layer of the mesoderm; v, nerve cord; t, notochord; w, Wolffian duct; 

 g, gastrocrele. 



with a definite lumen, which is here in the form of an irregular 

 crack between the outer and inner cell layers. 



Above this class the mesoderm appears first in the form of 

 an irregular cell layer which starts at the sides of the noto- 

 chord and invades the space between ectoderm and endoderm. 



