226 



ZOOLOGY. 



parts the dorsal part, consisting of mesoblastic somites ; a 

 continuous layer in contact with the hypoblast the splan- 

 chnic mesoblast ; and a layer in contact with the ventral 

 half of the epiblast fhe somatic mesoblast I "Ry t.hi fima 

 a transverse section shows us all the broad features of a 

 section across the intestinal region of the adult (compare 

 figs. 115, B, and 105.) 



Meanwhile, the larva has been lengthening greatly, new 

 myocoelomic pouches being added behind as this growth 

 continues. 



Mouth and anus arise as pits of epiblast (called respec- 



Fig. 115. DIAGRAMMATIC TRANSVERSE SECTIONS OF A LARVA 

 OF AMPHIOXUS. 



In A, the neural plate has completely rolled up to form the spinal cord ; 

 beneath it the nptochord is distinct ; beneath this is the mesenteron. 

 The myocoelomic pouches are pushing their way between these 

 structures and the epiblast, on either side. 



tively stomodaeum and proctodseum), that first abut against 

 the blind ends of the mesenteron, and then open into it 

 (compare fig. 121, showing the same in the frog). The 

 other communications between alimentary canal and 

 exterior, viz. the gill-slits, arise in a different way, as 

 pouch-like outgrowths of the mesenteron (fig. 116, B). The 

 walls of these pouches consist, of course, of hypoblast and 

 splanchnic mesoblast. They grow out into the coelom 

 until they touch its opposite wall, i.e. the body wall, con- 

 sisting of epiblast and somatic mesoblast. In doing so they 

 of course obliterate the coelom locally, and so this con- 

 tinuous space becomes divided up into dorsal, lateral, and 

 ventral canals, as previously explained (chap, xvi., 4). 



