ORIGIN OF THE MESODERM 



63 



the endoderm cells along the inner and outer edges of the sheet of 

 mesoderm. 



In early stages and in the anterior part of the embryo this pro- 

 liferation process may alone be in evidence, so that in place of a broad 

 continuous sheet of mesoderm there are found two narrow strips — 

 one (Fig. 38, C, mes') arising from the endoderm just external to the 

 notochordal region and the other (mes") arising from the endoderm 





end. 



y.n 



Fig. 38. — Three transverse sections through an embryo of PrisHurus (Stage B, Balfour), 

 illustrating the origin of the mesoderm. (After C. Rabl, 1889.) 



Section A, through the posterior half of the embryo ; B, through the middle ; C, through 

 the anterior half, ect, ectoderm ; end, endoderm ; mes, mesoderm ; y.n, yolk nuclei. 



peripherally. The two strips are known respectively as the axial 

 (Riickert ; or Gastral, Rabl) and the peripheral mesoderm (Ruckert ; 

 or Peristomal mesoderm, Rabl). 



Much discussion has centred round this double origin of the 

 mesoderm and attempts have been made to distinguish axial and 

 peripheral mesoderm in holoblastic forms including even Amphioxus, 

 thus infringing one of the chief canons of embryological science — 

 that developmental phenomena in the higher forms are to be ex- 

 plained by those of the lower and not vice versa. 



