Cut upon Regeneration in Gunda ulvse. 



365 



restoration of the tail belongs to the body as a whole, while that for 

 restoring the head is an entirely independent one, which may or may not 

 be localised in some part of the body, notably the anterior end. Probably 

 head-formation is dependent on the presence of some specific substance 

 which in Leptoplana and Gunda is localised in the anterior end, and in 

 Planaria is found throughout the length of the body. It would be of great 

 interest and possibly of great importance if a comparison of the relative 

 number of ganglia found in the nerve trunks could be made in these three 

 genera, as it might produce some evidence for or against the suggestion that 

 the head-forming substances may be localised in the central nervous system. 

 At present Morgan's (6) statement that pieces of Planaria maculata entirely 

 devoid of nervous system are capable of complete regulation rather bears 

 against the idea that the nervous system is of such great importance in 

 regeneration. The facts presented in this paper do not justify a full dis- 

 cussion of Child's theory of the axial gradient ; all that need be said is that 

 to make this work coincide with his hypothesis the axial gradient of G. ulvce 

 must be assumed to slope very steeply at the anterior end. Probably the 

 idea that there is a sudden change in rate of chemical actions in G. ulvce as 

 one passes backwards from the anterior end is not antithetic to the idea of 

 the localisation of some specific enzyme in the front of the body. 



Finally it may be noted that this work has disproved the suggestion made 

 by Hallez (4) that Triclads and Polyclads each have their own method of 

 regeneration, for G. ulvce, which is a marine Triclad, behaves like the Polyclads 

 in its method of the restoration of the head. 



Summary. 



1. Kegeneration of posterior parts is independent of the presence of the 

 cerebral ganglia. 



2. Lateral regeneration behind the level of the ganglia is independent of 

 their presence. In front of the level of the ganglia at least one complete 

 ganglion must be present for regulation to occur. 



3. Anterior regeneration only occurs if the piece contains about two-thirds 

 of both ganglia. 



4. Heteromorphic heads are formed by short head-pieces where the cut 

 has passed through the ganglia. 



5. G. ulvce differs from most other Triclads and corresponds to Polyclads 

 in its mode of regeneration. 



2 E 2 



