DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 191 



Heads inhibited by the physiological factor to the less extreme terato- 

 morphic forms such as Figure 63, E, often undergo a secondary transfor- 

 mation into forms like Figure 72, C, with resorption of the original 

 cephahc lobes and development of new lobes in the normal position and 

 of two eyes, also normally placed. Here the median head region, not rep- 

 resented in the primary development, appears secondarily, and a normal 

 head, except for the primary median eye, results. In other cases all three 



Fig. 72, A-I. — Secondary modifications of head form, differential conditioning, and 

 differential recovery in Dugesia. A, B, differential recovery of anophthalmic heads; C, differ- 

 ential recovery of teratomorphic head; D, reduction of head and loss of preocular region in 

 low concentrations of alcohol and ether; E, F, secondary development of median region under 

 the same conditions that produced the primary effect, D (differential conditioning); G, ap- 

 proach to normal form after return to water; H, reconstitution of head in alcohol, ether, and 

 low temperature; /, secondary development of head in alcohol and ether (differential condi- 

 tioning) and with rise in temperature (differential acceleration) (in part from Child, 1921c). 



eyes may be more or less fused, that is, the head does not develop beyond 

 the teratophthalmic condition. 



Still more extreme secondary modifications of head form appear with 

 differential conditioning to, and recovery from, effects of external inhibit- 

 ing agents. When intact animals are placed in low concentrations of ethyl 

 alcohol or ether, ^ the head decreases in size relatively to the body, and 

 the preocular region often undergoes more or less complete reduction by 

 gradual resorption or by disintegration in the course of 2 or 3 weeks 

 (Fig. 72, D). At about this time or somewhat later new growth of the 

 median region may begin in the same concentration in which resorption 



' .Alcohol 1-1.5 per cent; ether 0.2-0.4 per cent. 



