1 82 



organisers: inducers of differentiation 



of the tips of the skeletal spicules against the ectoderm in the region 

 of the ciliated band is necessary for the formation of the arms. 

 The ectoderm and skeleton are therefore mutually dependent in 

 the formation of the arms. 



To'p. 







Fig. 86 

 Eye development in Amphibia. Above, early neurula showing neural plate 

 (Mpl.) and limits of presumptive eye-rudiments (Au.). Below, left, section of 

 early optic cup, with tapetal (Tap.) and retinal (Ret.) layers, and epidermis 

 proliferating to form the lens rudiment (L.). Below, right, eye at onset of 

 functional stage. C. cornea. The central portion of the lens has differentiated 

 into lens fibres. (From Mangold, Naturwiss. xvi, 1928, figs, b, e,f.) 



It should be noted that under the influence of abnormal environ- 

 mental agencies, the course of local differentiation may be markedly 

 modified. One example is the formation by frog embryos, markedly 

 retarded by being kept in solution of KCl, of an almost solid 

 neural tube, recalling that normally found in the development 

 of PetrotJiyzon and Teleosts. Another, of extreme interest, is the 



