8G 



AMPHIOXUS. 



the 



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

 1 he 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 

 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 

 out the splanchnoccelic wall from 

 all contact with the external epi- 

 dermis. The splanchnoccel becomes 

 the body cavity, or ccelom, of the 

 adult. 



It is interesting to note that 

 even at this stage, when the larval 

 development is completed, all the 

 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 

 various regions being brought about 

 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 



trans - 

 young 



FlG. 43. — Diagrammatic 

 verse section across a 

 Amphioxus immediately after 

 the completion of the larval 

 period. The section is taken 

 at a level between the atrial 

 pore and the anus. (After 

 Hatschek.) 



A, dorsal aorta. CH, noto- 

 chord. CM, myoccel. CS, 

 splanchnoccel. CU, cutis layer. 

 DF, cavity of dorsal fin. EP, 

 epidermis. US, skeletogenous 

 layer. I, spinal cord. MF, 

 muscle-fascia layer. ML, myo- 

 tomic muscle. V, subintestinal 

 vessel. VF, cavity of ventral fin. 

 (Compare also Figs. 40 and 41.) 



