THE MESOBLASTIC SOMITES. 



85 



The cavity of the protovertebra is sp oken of as a myoccel 

 (Fig. 42, cm) ; and at a stage when five primary gill-slits are > 

 present {cf. Fig. 36) t he myoccels of each pair of protovertebra) ^)C 

 -communicate with each other above the spinal cord (Fig. 42). 

 The outer or parietal wall of the protovertebra is very thin, and 

 closely applied to the epidermis : it gives rise to the cutis, or 

 connective tissue basis of the skin, and may be spoken of as the 

 cutis layer (Fig. 42, cu). The 

 inner or notochordal wall of the 

 protovertebra, as already noticed 

 (p. 67), thickens very greatly, and, 

 though still remaining only one cell 

 thick, becomes converted into the 

 myotomic muscles (Fig. 42, ML). 

 The lower or visceral wall of the 

 protovertebra, like the parietal wall, 

 is thin, and is in contact with the 

 dorsal wall of the alimentary canal. 



The cavity of the lateral plates, 

 or splanchnoccel (Fig. 42, cs), is 

 continuous from end to end of the 

 body, through absorption of the 

 septa between the successive so- 



d— ML 



CM 



FlG. 42.— Diagrammatic trans- 

 verse section across the 

 intestinal region of an Am- 

 phioxus larva with five 

 primary gill-slits : cf. Fig. 3G. 

 (After Hatschek.) 



CH, notochord. CM, myoccel. 

 CS, splanchnoccel. CU, cutis 

 layer. EP. epidermis. I, spinal 

 cord. ML, myotomic muscle. 

 TI, intestine. V, subintestinal 

 blood-vessel. 



cs 



mites ; it is also continuous from 

 side to side across the mid-ventral 

 plane. The walls of the splanch- 

 noccel are thin ; the outer, or 

 parietal layer, is in contact with 

 the ventral epidermis, while the inner or splanchnic layer 

 clothes the sides and ventral wall of the alimentary canal. 



In the later stages important changes occur in these rela- 

 tions, and the condition immediately after the completion of the 

 larval period is shown in Fig. 43. 



The myoccels now extend ventral wards much further than 

 before, so that the parietal layer of the splanchnoccel (Fig. 43, 

 ) no longer touches the epidermis. The median dorsal and 

 ventral parts of the myoccels have separated off as the compart- 

 ments, df and VF, of the dorsal and ventral fins, which are 

 now prominent structures. 



The ventral or splanchnic wall of each myoccel is folded to 



