EMBR YON 1C DE VEL OPMENT. 



123 



nerve-cord, together with the internal sheath or fascia 

 of the muscles of the myotome, arises from the walls of 

 a pouch-like diverticulum of myoccel which grows up be- 

 tween the muscles and the notochord and nerve-cord. (Cf. 

 Figs. 67 and 68.) The myoccel also grows downwards 

 between the somatic layer of the peritoneum and the ecto- 

 derm (Fig. 68). According 

 to Hatschek the dorsal and 

 ventral fin-spaces are also 

 derived from the myoccel. 3 



The diverticulum of the 

 myoccel which has just been 

 described is known as the 

 sclerotome, since it gives rise 

 to the fibrous sheath of the 

 notochord and nerve-cord, 

 which (i.e. the sheath) in 

 most of the higher forms 

 becomes replaced by carti- 

 lage, and finally by bone. 

 In the craniate Vertebrates Fig 68 _ Scheme of a transverse 



the SclerotOme arises as a section through region between atriopore 



and anus, of a young Amphioxus shortly 

 Solid proliferation of cells after the metamorphosis. (After HAT- 



from the visceral wall at the ;> porsal fin . space _ ^ MyocoeU 



base of the SOmite. This 5C - Sclerotome. ao. Aorta, al. Intestine. 



i.m. Intercoelic membrane, s.i.v. Sub-in- 

 solid proliferation is Ull- testinal vein. sp. Splanchnocoel. v.f.c. 



doubtedly a modification of Ventral fin ' space - 

 a hollow diverticulum, involving, as it does, only the 

 visceral wall of the somite, precisely as we find it in 

 Amphioxus. 4 (Cf. Fig. 33.) 



On their outer surface the muscles of the myotomes are 

 not provided with a sheath of connective tissue (fascia), 

 standing, in this respect, in contrast to the condition 

 which obtains in the Craniota. 



