DYNAMICS — MOKPHOGENESIS AND INHERITANCE 419 



are normal as regards position of cephalic lobes and preganglionic 

 development show only a single eye. 



The anophthahnic head. This represents a still more extreme 

 stage of differential inhibition, the eyes and usually the cephalic 

 lobes being absent and the ganglionic mass remaining rudimen- 

 tary. Externally the anophthalmic head usually remains with- 

 out distinguishable organs of special sense (figs. 10, 11), though 

 sometimes a partially double or wholly single cephalic lobe 

 appears at. the tip (figs. 12, 13), Except in some cases of very 

 slight development of new tissue, behavior of anophthalmic 

 animals differs distinctly from that of acephalic forms, being more 

 like that of forms with more highly developed heads, and section 

 shows at least a rudimentary ganglion. As indicated in figure 14, 

 the anophthalmic head represents almost complete inhibition 

 of head development. 



The acephalic for7n. Here development of a head is com- 

 pletely inhibited, the cut surface contracts, the wound is merely 

 filled in with cells, and healing occurs without outgrowth (figs. 

 15, 16). Even here, however, differences in the contraction of 

 cut surface and amount of new tissue appear in different head- 

 less pieces, as figures 15 and 16 indicate. Such differences are 

 of course dependent on the degree or completeness of inhibition 

 of outgrowth. 



This series of head-forms from normal to acephalic evidently 

 represents different degrees of differential inhibition. The differ- 

 ential between median and lateral is clearly marked, the median 

 region being most susceptible and so most inhibited and sus- 

 ceptibility decreasing laterally. The longitudinal differential also 

 appears to some extent, the ganglionic region being the most sus- 

 ceptible region, but the difference between it and the pregangli- 

 onic region is slight. In consequence of these susceptibility 

 differentials, inhibition of head-development involves first the 

 median ganglionic region and then progressively the anterior and 

 lateral regions, until head-development is completely inhibited. 

 It must remembered that the differential susceptibility deter- 

 mining the different head-forms is that of the earlier stages of 

 head-development, and that the susceptibility relations during 



