m 



AMPHIOXrS. 



form a pouch, whicli extends npwarclp. between tlie myotome on 

 the outei' 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 (Kg. 43, hs) gives rise to the 



skeletal connective tissue, which 

 invests the notochord and the spinal 

 cord. The^_cavitvof__the_-peucb 

 becomes ultimately obl iterate d by 

 gro wth of the connective t issue, 

 ex cept in the anter ior three or four 

 segments_ofthe body. 



The splanchnoccfil (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 ; w hile the myoccel in it s turn, 

 nwinjorto itfi vent ral extension , shuts 



mit _the Rp 1«nf.>|Tinf>nRliri wall from 



all c ontact with the ext ernal epi- 

 dermis. The s planchnocoel beco mes 

 th e body cavity^ or coelom, of the 

 adult. 



It is interesting to note that 

 even at this stage ^ w hen the larval 



Fig. i3. — Diagrammatic trans- 

 verse section across a young 

 Amphioxus immediately after 

 the completion of the larval 

 period. The section is taken 



at a level between the atrial , , — , . — =i - — = 7= — ,, 



pore and the anus. (After development IS completed, all th e 



Hatschek.) p arts of the body are, as js _J^e 



A, dorsal aorta. CH, noto- earlier stages already noticed in 



chord. CM, myoccel. CS, ° •' 



splanchnocoel. CTJ, cutis layer. thlS reSPCCt, madpi iin of Cpithfilial 



DP, cavity o( dorsal fin. EP, , . . * t--sa»*— 



epidermis. HS, skeletogenous laYP[rgwlnf^>i in e acll CaSO are but 



layer. I, spinal cord. MP, ■ -r — 



muscle-fascia layer. ML, myo- O ne Cell thicg : the Complications in 



tomio muscle. V, subintestinal - , ^ . ■*■ 



vessel, vp, cavity of ventral fln. vai'ious TBgions being brought about 



(CompaiealsoFigs.40ana41.) . 



by differences m 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 



