86 



AMPHIOXUS. 



DF 



CM 



form a pouch, which, extends upwards, between the myotome on 

 the outer side, and the notochord and spinal cord on the inner 

 side. The outer wall of this pouch (Fig. 43, MF) becomes 

 the fascia covering the inner surface of the myotome ; while 

 the inner wall of the pouch (Fig. 43, HS) gives rise to the 



skeletal connective tissue, which 

 invests the notochord and the spinal 

 cord. The cavity of the pouch 

 becomes ultimately obliterated by 

 growth of the connective tissue, 

 except in the anterior three or four 

 segments of the body. 



The splanchnoccel (Fig. 43, cs) 

 undergoes but slight modification. 

 It extends further dorsalwards than 

 before, and almost completely sur- 

 rounds the alimentary canal, cut- 

 ting out the myoccel from its former 

 share ; while the myoccel in its turn, 

 owing to its ventral extension, shuts 

 v out the splanchnocoelic wall from 

 all contact with the external epi- 

 dermis. The splanchnocoel becomes 

 trans- tne body cavity, or coelom, of the 

 adult. 



It is interesting to note that 



MF 

 HS 



Ti 



CS 



VF 



FiG. 43. Diagrammatic 



verse section across a young 



Amphioxus immediately after 



the completion of the larval 



period. The section is taken ev en at this stage, when the larval 



at a level between the atrial -. , . . , . ^ -.-. , n 



pore and the anus. (After development is completed, all the 



Hatschek.) parts of the body are, as in the 



earlier stages already noticed in 

 this respect, made up of epithelial 

 layers, which in each case are but 

 one cell thick ; the complications in 



tomic muscle. V, subintestinal . . . , 



vessel. VF, cavity of ventral fin. various regions being brought about 



A, dorsal aorta. CH, noto- 

 chord. CM, myocoel. CS, 

 splanchnocoel. CU, cutis layer. 

 DF, cavity of dorsal fin. EP, 

 epidermis. HS, skeletogenous 

 layer. I, spinal cord. MF, 

 muscle-fascia layer. ML, myo- 

 V, subintestinal 



(Compare also Figs. 40 and 41.) 



by differences in the shapes of the 



cells at different places, together with foldings of the walls of 

 the several cavities. 



The origin of the connective tissue is not determined with 

 certainty. Hatschek considers that it is at first of a gelatinous 

 nature, probably formed by excretion from, and between, the 



