PACTS AND FACTORS OF DEVELOPMENT 27 



comes a gastrula (Fig. 11, K, L), in which 

 at first two, and later three, groups or layers 

 of cells may be recognized; the outer layer, 

 which is formed from cells nearest the upper 

 pole of the egg, is the ectoderm; the inner 

 layer, or endoderm, is formed from cells near- 

 est the lower pole; a middle layer, or group 

 of cells, the mesoderm, is formed from cleav- 

 age cells which in vertebrates lie between the 

 upper and lower poles. 



som 



som. 

 mes 



Fig. 13, A and B. Two Later Stages ik the Development 

 OF ArrvpMoxus, showing the elongation of the embryo in the 

 antero-posterior axis (a p), and formation of the somites 

 {som) ; neural groove (ng) and neural tube (rai) ; ect, ecto- 

 derm; mes, mesoderm; ac, alimentary canal. (After Hatschek.) 



