THE DEVELOPMENT OF AMPHIOXUS 53 



the cut is layer of the integument. The blood cells and many 

 of the important blood-vessels appear to be derived from the 

 skeletogenous layer of the sclerotome. 



In the adult the lateral plate forms only the lining membrane 

 of the coelom, its somatic layer, the body wall, its splanchnic 

 layer, chiefly the wall of the gut outside the endodermal 

 epithelium. The ventral musculature develops later and 

 appears to be derived from the parietal (somatic) layer of the 

 lateral plate. 



Among the Craniates the sclerotome is formed from a mass 

 of mesenchyme cells, a tissue which is noticeably lacking in 

 Amphioxus. It remains an open question whether in this 

 organism the formation of the sclerotome by a folding of the 

 myocoelic wall, and the correlated paucity of mesenchyme cells, 

 is a primitive or secondary character. 



11. The Nephridia 



The development of the nephridia is very incompletely 

 known (Legros, Goodrich). They appear in connection with 

 each gill pouch, as these are first marked out (Fig. 14, A}, as 

 groups of mesoblast cells along the outer faces of the endo- 

 dermal gill pouches, lying at the base of a narrow coelomic groove 

 in this region. These mesoblast cells soon form a definite 

 blind tube extending posteriorly and ventrally from the 

 ccelom toward the gill pouch. The mesodermal cells compos- 

 ing the small nephridial rudiments become differentiated, for 

 the most part, as clear cubic cells, but a group of soknocytes 

 is distinguished as a small collection of elongated granular 

 cells in the neighborhood of the coelomic opening of the tubule. 

 Then the lower end of the nephridial tube fuses with the dorsal 

 wall of the gill pouch, just posterior to the region of the devel- 

 oping tongue bar. Finally it opens into the gill pouch, about 

 the time this becomes perforated externally, and at the same 

 time its coelomic opening is closed and the solenocytes elongate 

 into the ccelom (Fig. 20). 



The first formed nephridia apparently become functional 



