92 



COMPARATIVE ANATOMY 



represents a chick embryo after about t,:^, hours incubation. The meso- 

 dermal somites, easily seen through the very thin ectoderm, appear as 

 pairs of denser or darker squarish blocks. 



Fig. 63.- — Stereogram of the anterior region of a vertebrate embryo showing the 

 segmentation of the mesoderm. The ectoderm has been removed from the left side 

 of the embryo, al, endoderm of aUmentary tube; c, coelom; em, epimere;/6, fore-brain; 

 hb, hind-brain; hm, hypomere; m, myotome; mb, mid-brain; mm, mesomere; n, neural 

 tube; nc, notochord; s, stomodeal region; sk, sclerotome; so, sp, somatic and splanchnic 

 walls of coelom. (From Kingsley.) 



The process of segmentation involves only the upper part of the 

 mesoderm. As segmentation goes on, the space between the two thin 

 and unsegmented layers becomes wider — a space already recognizable 



Fig. 64. — Diagrams showing embryonic origin of pronephric tubules. A, earlier 

 stage; B, later, c, coelom; d, pronephric tubule and duct; e, epimere; h, hypomere; 

 w, mesomere (cross-lined); my, myotome; «, nephrostome; so, somatic layer of hypo- 

 mere; sp, visceral (splanchnic) layer of hypomere. (From Kingsley, after Felix.) 



as the coelom bounded externally by a somatopleure consisting of ecto- 

 derm and the outer sheet of mesoderm, and internally by a splanch- 

 nopleure consisting of endoderm and the adjacent layer of mesoderm. 

 The mesodermal layers upon either side grow down to the mid-ventral 



