EMBRYONIC DEVELOPMENT. 



121 



.n r 



mye 



nexion with the archenteron, they commence to extend 

 dorsally 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. Fi S- 66. Transverse section through 



the middle of the body of an embryo, 



The latter gradually grOWS with ten pairs of somites, to show the 

 j i . -i , closure of medullary tube and the dorsal 



downwards, until it meets and ventra i extension of the mesodermic 

 its fellow of the Other side ; somites. (After HATSCHEK.) 



al. Alimentary canal, c/i. Notochord, 

 the tWO fuse together, and in the cells of which vacuoles have com- 



so the cavity is made con- ^S *?: 



tinUOUS from Side tO Side, the cells forming the inner wall of the 



somite, my.c. Myoccelomic cavity. 



below the intestine. 



When this occurs, the primarily single cavity of each 

 archenteric pouch becomes divided into two portions ; 

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

 and a ventral portion, the ccelom, by a transverse partition, 

 which arises through a fusion between the parietal and 



* Already in the embryo shown 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 locomotion 

 effectually replaces the primitive ciliary locomotion, although the cilia persist 

 to a late stage. 



