192 PATTERNS AND PROBLEMS OF DEVELOPMENT 



or disintegration occurred earlier, and after about a month in the solution 

 head forms like Figure 72, £ and F, have developed. Apparently the 

 median region gradually acquires a greater ability to grow and develop 

 than lateral regions. After such forms are returned to water, the head 

 may gradually approach normal form, but even after two months or more 

 the preocular region is still larger than normal (Fig. 72, G). 



Very similar modifications of head form occur in reconstitution of heads 

 on pieces under external inhibiting conditions. In ether, alcohol, chlore- 

 tone, and other anesthetics heads like Figure 72, H, appear frequently: 

 in these the growth of new tissue is largely inhibited; but in the course of 

 8-10 days a single median eye appears, often in the pigmented old tissue. 

 With some further development such heads would doubtless be terato- 

 morphic. After 2 weeks or more in the solution further growth may begin, 

 resulting in heads with elongated median region, and often two new eyes 

 and cephalic lobes appear in normal position (Fig. 72, /). Here the me- 

 dian regions not represented in earlier stages gradually develop as condi- 

 tioning proceeds and become overdeveloped relative to other parts of the 

 head. Essentially the same head forms appear in differential recovery 

 after exposure for a week or two to the same agents. Secondary modifica- 

 tions of the same type have also been obtained in the differential accelera- 

 tion of development resulting from change to high temperature after 

 conditioning to low. Pieces from a low-temperature stock (conditioned 

 to 3°-5° C.) reconstituting at 4°-8° C. produce little new tissue, and most 

 of them remain acephalic or like Figure 72, H, even after 3 months. 

 Brought to room temperature (2o°-24° C.) many pieces develop heads 

 with median elongations Hke Figure 72, /, or without median eye. These 

 cases may involve differential conditioning to the higher temperature, as 

 well as differential acceleration of development. The occurrence of these 

 forms with various anesthetics and with change from low to high tem- 

 perature makes it probable that they, like other modifications of planarian 

 head form, are expressions of differential susceptibility and not specific 

 for any particular agent. Doubtless they can be produced with many 

 other agents which permit relatively rapid differential conditioning or re- 

 covery. 



THE PLANARIAN HEAD: INTERPRETATIONS AND SUGGESTIONS 



It is a point of considerable importance that both the physiological in- 

 hibiting factor and the external inhibiting agents give the same inhibition 

 scries of head forms in Dugesia, a continuous graded series of medio- 



