52 OUTLINES OF CHORD ATE DEVELOPMENT 



and the axial structures notoch'ord and nerve cord, to the 

 region of the dorsal fin (Fig. 19, B). This is the sclerotome; its 

 inner or axial layer is called the skeletogenous layer and gives 

 rise to dense fibrous connective tissue forming the notochordal 

 and neural sheaths; its outer or myotomal layer remains thin 

 and forms the fascia covering the muscular protovertebra : 



FIG. 19. Diagrams of transverse sections through Amphioxus larvae. A. 

 Through the body region of a larva with five gill slits, showing separation of 

 myocoel and splanchnocoel (coelom). B. Through the region between atriopore 

 and anus of young individual, shortly after metamorphosis, showing relations of 

 sclerotome. After Hatschek. a, Dorsal aorta; c, coelom (splanchnocoel) ; ch, 

 notochord; d, dermatome; df, dorsal fin cavity; e, epidermis; i, intestine; me, 

 myocoel; mp, muscle plate (myotome); n, nerve cord; s, sclerotome; v, sub- 

 intestinal vein; vf, ventral fin cavity. 



the cavity between the two layers is soon obliterated by an 

 accumulation of connective tissue. There is no fascia on the 

 outer surface of the protovertebra or myotome such as is found 

 in Craniates. The second or dermal fold (dermatome) of the 

 myoccelic wall, extends downward and outward between the 

 somatic layer of the lateral plate and the ectoderm (Fig. 19, B), 

 where its inner and outer layers come into apposition and form 



