364 Miss D. J. Lloyd. Influence of the Position of the 



the ganglia causes the production of defective heads. When ganglia and 

 nerve roots are completely removed head-formation is entirely inhibited. 

 The behaviour of Planaria dorotocephcda was found by Child (2) to be 

 in direct contrast to this. Planaria dorotocephala under normal condi- 

 tions regenerates the anterior end completely from any point of section. 

 It is only when regeneration is suppressed by the addition of anaesthetics 

 or potassium cyanide that defective heads are produced essentially similar 

 to those obtained in Zeptoplana and Gunda ulvce, i.e., heads with three to 

 four eyes, or with a single median eye and with the auricles approxi- 

 mated or even fused. With increasing defect of the external appearance 

 Child has shown that there is increasing defect in the regeneration of the 

 ganglia, e.g., in teratomorphic (Child) heads, with single median eye and 

 fused auricles the cephalic ganglia are partially or completely fused ; 

 in pieces in which there is no head-regeneration there is similarly no 

 regeneration of the cephalic ganglia. In fact so closely does the 

 parallel run that it seems almost justifiable to assume that in Zeptoplana, 

 Planaria, and Gunda head-regeneration is dependent on the presence of the 

 central nervous system, and that the difference between them is found in 

 the greater power of regeneration possessed by the central nervous system 

 of Planaria. Planaria has another characteristic in which it contrasts 

 strongly with the two other genera, and that is in its power of asexual 

 reproduction. It seems quite likely that asexual reproduction and the high 

 capacity for head-formation are both determined by the power of growth and 

 regeneration of the central nervous system. 



In G. ulvas the condition is found that the nerve cords can exhibit restora- 

 tion by backward and forward growth after a cut made at any part of their 

 length, quite independently of the ganglia. The ganglia themselves appear 

 incapable of restoring lost parts, unless one complete ganglion is present, and 

 the restoration of complete heads only occurs if the ganglia are restored. 



Considering the inability of animals with badly damaged central nervous 

 system to regenerate heads, the production of heteromorphic heads on 

 short anterior pieces is of great interest. These pieces contain only about 

 one-third of the cerebral ganglia, yet the heads which they regenerate at 

 the posterior end are complete with trifid gut, eyes and auricular processes. 

 It will be remembered that large posterior pieces with defective cerebral 

 ganglia failed to regenerate heads on the oral pole. 



In Planaria maculata Morgan has obtained heteromorphic heads at any 

 point of section, provided that the fragments which he took were sufficiently 

 short. 



From these considerations it seems possible that the mechanism for the 



