566 



CEPHALOCHORDA. 



in contact with the intestinal wall ; this is known as the sclera-layer 

 and this portion of the coelom as the sderotome (#). At the point at 

 which it passes over into the cutis-layer, there is a large boundary 

 cell (4) which, according to BOVERI, is probably to be regarded as a 

 primitive genital cell. 



In the lateral plates also, a parietal layer (5} can be distinguished 

 from a median (visceral) layer. The parietal layer (somatopleure) at 

 first lies on the inner side of the ectoderm ; the visceral (splantfmo- 



FIG. 301. Transverse section through 

 a young . A mphioxus, immediately 

 after metamorphosis, through the 

 region between the atriopore and the 

 anus (after HATSCHBK). 



FIG. 302. Diagrammatic rendering 

 of the same section. 



A, epidermis; B, medullary tube ; C, chorda ; D, aorta; A', intestinal epithelium; 

 F, sub-intestinal vessel. 1, cutis-layer ; 2, muscle-layer ; 3, fascia-layer ; 4, outer 

 chorda-sheath ; 4', muscle-septum ; 5, gastral continuation of the skeletogenous layer 

 (intercoelic membrane) ; 6, somatopleure ; 7, splanchnopleure ; 7, myocoele ; / 

 dorsal, /, ventral fin-cavity ; //, splanchnocoele. 



pleure) forms the dorsal mesentery, in connection with which the aortae 

 develop later, and surrounds the intestine and the sub-intestinal vessel, 

 running along the ventral surface of the latter. This layer yields the 

 unstriped muscle-layer of the alimentary canal. 



In later stages (Figs. 301, 302) the muscle-plate becomes com- 

 pletely separated from the chorda dorsalis and the nerve-tube, in 

 consequence of the development of an outgrowth of that part of 

 the myocoele termed the sclera-layer which grows up from below 



