FACTS AND FACTORS OF DEVELOPMENT 29 



cate cellular rod (Fig. 13, ch), which then in 

 higher vertebrates becomes surrounded suc- 

 cessively by a fibrous, a cartilaginous, and a 

 bony sheath. And so one might go on with a 

 description of all the organs of the body, each 

 of which begins as a relatively simple group 

 or layer of cells, which gradually become more 

 complicated by a process of growth and dif- 



FIG. 13. CROSS SECTION OF Amphioxus LARVAE IN SUCCESSIVE 

 STAGES OF DEVELOPMENT. A, through a larva similar to 11A; 

 B and C, of a larva similar to 11B; D, of a still older larva; 

 ect, ectoderm; ent, endoderm; mes, mesoderm; ch, notochord; 

 np, neural plate; gc, gastrocoel; ac, alimentary canal; coel, 

 coelom. 



