EMBIt ] -OXIC DE J 'EL OPMEXT. 



\2\ 



nexion with the archenteron, thev commence to extend 

 dorsallv and ventrally between the ectoderm and the in- 

 ternal organs (Fig. 66). Meanwhile the cells forming the 

 inner or visceral wall of the pouch adjacent to the noto- 

 chord elongate transversely and longitudinally, and begin 

 to form the plate-like muscle-fibres of the myotome. The 

 cells producing these fibres 

 are arranged in such a way 

 that each of them gives rise 

 to a muscle-fibre extending 

 from the anterior to the pos- 

 terior limit of a myotome.* 

 The muscles are at first 

 closely approximated to the 

 notochord and project freely 

 into the cavity of the pouch. 

 The latter gradually grows 

 downwards, until it meets 

 its fellow of the other side ; somites. (.After hatschek.) 



.:/. Alimentary canal, ch. Notochord, 



Fig. 66. — Tr.insverse section through 

 the middle ol the body o\ an embr\o, 

 with ten pairs of somites, to show the 

 closure of medullar}" tube and the dorsal 

 and ventral extension of the mesodermic 



the two fuse together, and 

 so the cavitv is made con- 



in the cells of which vacuoles have com- 

 menced to form. l.m. Commencing for- 

 mation of longitudinal muscle-plates from 

 tinUOUS from side to side, 'he cells forming the inner wall of the 



somite, ir.x.c. Mvoccelomic cavitv. 



below the mtestme. 



When this occurs, the primarily single cavity of each 

 archenteric pouch becomes divided into two portions ; 

 namely, a dorsal portion, the somite proper or nivocivl, 

 and a ventral portion, the cirhvu. b)- a transverse partition, 

 which arises through a fusion between the parietal and 



* Already in the embryo show-n in Fig. 63 C and even at a somewhat ear- 

 lier stage, the muscles are so far developed that the body can be bent and 

 jerked. By the time the mouth has broken through, muscular loccmolion 

 efiectually replaces the primitive ciliary locomotion, although the cilia persist 

 to a late stage. 



