Tire MESOBLASTIC SOMITES. 



85 



The cavity of the protovertebra is spok en of as a myocoel 

 (Fig. 42; cm) ; and at a stage when five primary gill^lits are 

 present (c/. Fig. 36) the myocoels of each pair of protoverte bree 

 co mmunicate with each other above the spinal cord (Fig. 42). 

 The outer ja r parietal wall of the protov ertebra ia very thin , and 

 closely ap plied to the epiderm is : it g ives rise to the cutis, or 

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

 j^T ifiij-rg yer (Fig. 42, cu). The 

 in ner or notochordal wall of the 

 protove rtebra, a s a,1rea,d y xi oticed 

 (p. 67), thic kens very greatly, and , 

 though still remaining only one cell 

 thick, bec omes converted into the 

 mv otomic muscles (Fig. 42, ml). 

 The lower or visceral wall of the 



proto verte bra, li ke the parieta l wall, 

 is thin , anci is in con tact with the 

 dorsal w all of the alimentary can al. 

 The cavity of the lateral plates, 

 or splanchnoco el (Fig. 42, cs), is 

 continuous i'rom end to end of the 

 body, through absorption of the 

 septa between the successive so- 

 mites ; it is also continuous from 

 side to side across_ the mid-vent ral 

 The walls of the splanch- 



FiG. 42. — Diagrammatic trans- 

 verse section across the 

 intestinal region of an Am- 

 phioxus larva with five 

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

 (After Hatschek.) 



CH, iiotochord. CM.myocctl. 

 CS, splauchnocccl. CIT, cutis 

 layer. EP. epidermis. I, spiiiiil 

 cord. ML, myotomic muscle. 

 TI, intestine. V, subintestinal 

 blood-vessel. 



plane. 



iioccbI 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 myocoels now extend ventralwards much further than 

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

 cs) no longer touches the epidermis. The median dorsal_ and 

 ventral partso f the myocoels 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 myocoeHs-iolded to 



