REPRODUCTION 



67 



of the original sheet (Figs. 58, 59.! J. At about the same time the dorsal 

 and thicker part of the mesoderm develops transverse fissures which 

 divide it into a series of paired blocks (somites) lying symmetrically 

 either side of the neural tube (Fig. 58). This segmentation begins in the 

 anterior part of the embryo and progresses backwards just as, in Amphi- 

 oxus, the mesodermal pouches are formed successively from anterior to 

 posterior. 



The process of segmentation involves only the upper part of the 

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

 and unsegmented layers on either side becomes wider — a space already 



Fig. 58. — 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 alimentary tube; c, coelom; etn, epimere;/6, fore-brain; 

 hb, hind-brain; km, 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.) 



recognizable as the coelom bounded externally by a somatopleure con- 

 sisting of ectoderm and the outer sheet of mesoderm, and internally by a 

 splanchnopleure consisting of endoderm and the adjacent layer of meso- 

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

 ventral region, carrying with them the coelom, and meet mid-ventrally 

 to form a double vertical layer, a ventral mesentery, extending from the 

 enteron to the outer body wall and separating right and left coelomic 

 cavities. (Fig. 58) 



The splitting of the original sheet of mesoderm extends so far dorsally 

 as to involve the somite which accordingly contains a more or less definite 

 cavity, the myocoele — "myo-" because the somite is mainly muscle- 

 forming. Shortly the somites become detached from the lower somatic 



