566 



CEPHALOCHORDA. 



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

 and this portion of the coelom as the sclerotome ($). 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 (•->) can be distinguished 

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

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



Fig. 301.— Transverse section through Fig.302.— Diagrammaticrendering 



a young Amphioxus, immediately of the same section. 



after metamorphosis, through tin- 

 region between the atriopore ami the 

 anus (after Hatschek). 



.1. epidermis; /!, medullary tube ; <\ chorda; h. aorta; E, intestinal epithelium; 

 F, sub-intestinal vessel. /. cutis-layer : .'. muscle-layer ; 3, fascia-layer ; 4, outer 

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

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

 dorsal, / /f 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 

 uristriped 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 sclent-layer which grows up from below 



