DIFFERENTIAL DEVELOPMENTAL MODIFICATION. Ill 263 



I -minute exposures to ultra-violet radiation of a certain wave length 

 range, show strong ventral curvature and are apparently cases of differen- 

 tial tolerance or conditioning, while those resulting from daily 2-minute 

 exposures are, at least in part, difTerentially inhibited forms with persist- 

 ent open blastopore. 



Bellamy's experiments show that the dorsal lip of the blastopore at 

 the beginning of gastrulation is highly susceptible to many agents and 

 that its median region is more susceptible than lateral regions, but they 

 do not indicate specific susceptibility to any of the agents used. Moreover, 

 they do not show whether, or to what extent, ectodermal differential 

 inhibitions of the head and dorsal region are direct effects of the agent on 

 the ectoderm or results of differential inhibition of the chorda-mesoderm, 

 the dorsal inductor (see chap. xii). The high susceptibiHty of the dorsal 

 lip, decreasing anteriorly and laterally, suggests that differential inhibition 

 of this region may be of primary importance in various modifications of 

 the ectoderm, but doubtless direct differential inhibition of the ectoderm 

 after induction has taken place plays a part in the modifications — for 

 example, in those of the head. 



By removal of membranes of axolotl and anuran blastulae and culture 

 in a modified Ringer solution partial or complete exogastrulation has been 

 produced. '^ In total exogastrulation no neural plate develops in the ecto- 

 derm, apparently because of absence beneath it of the entomesodermal 

 inductor, but the evaginated entomesoderm differentiates without evi- 

 dence of retardation or differential inhibition (see pp. 463-65) . Exogastru- 

 lation also occurs in some cases in water after removal of membranes, ap- 

 parently in consequence of collapse of the blastocoel; but in salt solution 

 all axolotl embryos become exogastrulae. From the data it appears prob- 

 able that a differential susceptibility is not primarily concerned in this 

 type of exogastrulation. The collapse of the blastocoel in the soft, highly 

 fluid axolotl embryo without membranes is probably the chief factor. 



The view that lithium has specific regional effects, differing at differ- 

 ent developmental stages and dependent on critical stages or phases in 

 development of the dorsal inductor region, and that action of external 

 agents in general on embryonic development shows many evidences of 

 specific regional effects differing at different stages has been advanced 

 recently in a number of papers by Lehmann.^ The evidence presented in 

 these papers, however, does not at present seem to provide adequate basis 



7 Holtfreter, 1931a, igs^d, e; see also Goertller, 1926. 



* F. E. Lehmann, 1933; 1934; 1936a, b; 1937a, b, c; 1938. 



