EMBRYONIC INDUCTORS AND ORGANIZERS 465 



work which is not in accord with it, as Huxley and De Beer (1934, P- 493) 

 have pointed out; and there are other points to be considered. The fact 

 that the presumptive neural-plate region of the exogastrulae undergoes 

 more stretching than presumptive epidermis indicates a difference of some 

 sort, though it may be a nonspecific gradient difference. Moreover, the 

 Ringer solutions that bring about exogastrulation may inhibit develop- 

 ment to some extent; various Ringer modifications inhibit planarian re- 

 constitution. If this is the case, the exogastrulae do not provide the final 

 proof of complete dependence of neural development on induction. 



The results with presumptive epidermis and neural plate cultured in 

 salt solution or in vivo in the coelom or eye cavity show that a piece of 



Fig. 160. — An exogastrula with mesentoderm (left) somewhat similar in form to an embryo 

 and connected with the irregular ectodermal mass (right) only by a slender strand (from 

 Holtfreter, 19336, Biol. Zbl., 53, H. 7, 8, Thieme). 



either may develop as epidermis, neural tissue, or chorda-mesoderm. 

 Holtfreter regards the eye cavity as having complex inductive capacities, 

 but the possibility is not yet excluded that the primary differences in 

 these tissues are nonspecific. The doubtful results of the temperature ex- 

 periments are not conclusive evidence against this view. In developing as 

 neural tissue ectoderm apparently undergoes a rise in physiological level, 

 relative to the other ectoderm, and apparently a further rise is necessary 

 for development as chorda-mesoderm. The isolated pieces, implanted in 

 coelom or optic cavity, are isolated from the gradient pattern of the em- 

 bryo and apparently under favorable conditions. Development of chorda- 

 mesoderm from almost all regions of the early gastrula, implanted in the 

 optic cavity, does not seem to be very different in principle from develop- 

 ment of a hydranth or head from any level of the hydroid or planarian 



