286 CHARLES RUSSELL BARDEEN. 



differentiation of a head has taken place otherwise than about the 

 tip of an intestinal branch connected with the main intestinal ap- 

 paratus of the animal. This intestinal branch may be termed the 

 axial gut because of its subsequent relations to the regenerated 

 worm. Lillie, in a valuable contribution to the subject of regen- 

 eration in planarians (op. cif., p. 141), states that "the intestinal 

 system regenerates in relation to the new external parts and not 

 vice versa, as maintained by Bardeen ; from which it follows that 

 the location of the new parts cannot be due primarily to the form 

 of the gut." As a matter of fact it is probable that mutual rela- 

 tions exist between the intestinal system of the animal and the 

 regenerating parts, each exerting specific influences upon the 

 growth of the other. 



After the head and pharynx have been formed, the processes 

 which serve to restore the worm to normal proportions become 

 very marked and, unless certain conditions prevail, such as lack 

 of size or attachment to another individuality (as shown in Fig. 

 1 2, B\ this is soon effected. Bilateral symmetry, however, may 

 arise in partially detached pieces containing neither head nor 

 pharynx. Morgan has made a special and valuable study of the 

 general gross relations borne by an isolated part to the regener- 

 ated individual. 1 



In small pieces isolated from sexually mature worms the re- 

 productive organs at first completely disappear, as shown above 

 in discussing regeneration in tail pieces. They are regenerated 

 only after the worm has reached a certain advanced stage of de- 

 velopment. In Planaria maculata, furthermore, the development 

 of the sexual organs seems to depend upon season. In Dendro- 

 ccelum lacteuin lijima found that after depositing the cocoons the 

 individuals die, but this is not true of specimens of P. inaculata 

 kept in captivity. It is probable that there is great variation in 

 different species of planarians in the time of development and the 

 stability of the reproductive organs. Schultze, as mentioned 

 above, gives an excellent account of the regeneration of the re- 

 productive organs in D. lacteum (pp. cit.}. 



1 Op. cit., and also "The Internal Influences that Determine the Relative Size of 

 Double Structures in Planaria higubris," BIOLOGICAL BULLETIN, Vol. III., p. 132, 

 1902. 



