8 



COMPARATIVE ANATOMY 



pleura, pericardium and muscles, as well as to almost the entire 

 excretory and reproductive apparatus. 



A space in the mesodermic tissue divides it into a parietal or 

 somatic layer, lying along the inner side of the ectoderm, and 

 into a visceral or splanchnic layer, which becomes attached to the 

 outer side of the endoderm (Fig. 6, A and B). The former, together 

 with the ectoderm to which it is united, constitutes the somato- 



mes 



D 



Fi<;. 7. DIAGRAMMATIC SECTIONS OF THK PKIMITIVK KNDODKKM AND ITS 



DERIVATIVES OF AMPHIOXUS (A C, AFTER GOTTK). 



From the dorsal part of the endoderm (unshaded) arise the notochcml (<//) and 

 the mesodermic somites (mrx) ; from the ventral part (shaded), the epithelial 

 lining of the gut. The entire endoderm in stages R and G thus shows four 

 hollow projections : a median (chordal) and paired lateral (mesodermal) out- 

 growths communicating at first with the larger ventral intestinal cavity (tlh), 

 but later becoming constricted off from the latter (D). The notochordal 

 outgrowth becomes solid, while the cavities in the mesodermal segments 

 represent the rudiments of the ctelome. 



pi cure, and the latter, together with the endoderm, the splanclmo- 

 plcure. The cavity separating these is the body-cavity or ccclomc, 

 and is lined by an epithelium. 



The ccelomc may arise as a segmen tally arranged series of 

 pouches (enteroccelcs) from the archenteron, in which case its lining 

 epithelium is at first continuous with the endoderm, as is most 

 plainly seen in Amphioxus (Fig. 7); or it may be formed 

 secondarily by a splitting (delamination) of the mesodermic tissue 

 (schizocasle). The former of these must be considered as the more 

 primitive. 



