1 66 EMBRYOLOGICAL TYPES 



these kinds is solely one of origin. After the separation of 

 the mesodermal somites and of the notochord, the lateral 

 edges of the endoderm grow over and meet to reform a roof 

 over the gut- cavity. 



The somites increase in size, and grow down between the 

 gut and the ectoderm on each side. Eventually they meet 

 beneath the gut and the wall separating them breaks down, so 

 that the ccelomic cavity of each somite communicates with 

 that of the corresponding somite on the opposite side of the 

 body. The layer of ccelomic wall or epithelium which touches 

 the endodermal wall of the gut is called the splanchnic layer, 

 that touching the ectoderm of the surface of the body is the 

 somatic layer. That part of the ccelomic wall which abuts 

 against the nerve-tube and notochord on each side becomes 

 thickened and gives rise to muscle-fibres forming the myotome : 

 one myotome to each somite on each side. The more dorsal 

 portions of the ccelomic cavity on each side, separating the 

 myotome from the outer (or cutis) layer, are called the myocoels ; 

 whereas the more ventral portion, into which the splanchnic 

 layer suspends the gut from above, is the splanchnocoel. The 

 myocoels become separated from the splanchnocoel of their 

 somite by a horizontal partition. The myocoels retain their 

 segmental arrangement, and remain separated by the septa 

 from the myocoels of the somites in front and behind. The 

 septa separating the splanchnocoels, however, break down, so 

 that there is a continuous splanchnocoelic or perivisceral cavity 

 from one end of the animal to the other. 



The myotomes soon begin to show the V-shape character- 

 istic of the adult, and the alternation in position between right 

 and left sides. 



In connexion with the mesoderm, there remain to be 

 described a pair of pouches which become nipped off from the 

 extreme front end of the wall of the gut. These are the 

 anterior head-cavities, or anterior gut- diverticula. They arise 

 symmetrically, but the right one soon occupies all the anterior 

 region of the embryo in front of the ist pair of myotomes, 

 and becomes the head-cavity. The left anterior gut-diverti- 

 culum remains small, and eventually acquires an opening to 



