CONDITIONS OF GAMETE FORMATION 387 



amount and uniformity of the food supply and the rate of growth. 

 There is no question that in P. maculata, as well as in P. doroto- 

 cephala, sexual maturity represents a condition of greater physio- 

 logical age than the asexual stage. 



The case of P. velata is somewhat different. Under the con- 

 ditions where it is usually found in nature, as well as in the labora- 

 tory, this form unquestionably grows old, ceases to feed, and 

 undergoes fragmentation in each generation without becoming 

 sexually mature. Apparently sexual reproduction has no place in 

 the life cycle of this species. If it were not for the fact that the 

 animal stops feeding and ceases to grow before fragmentation 

 occurs we might believe that the life cycle of the individual is sim- 

 ply interrupted as in P. dorotocephala and in many plants by the 

 agamic reproduction, but as a matter of fact the period of develop- 

 ment and growth is apparently completed before fragmentation 

 begins. Thus far it has not been possible to induce sexual maturity 

 experimentally in this species. It seems probable, however, that 

 certain of the feeding experiments already described afford a clue 

 to the understanding of this case. It was pointed out that the 

 length of the growth period and the amount of growth before 

 fragmentation differ very widely with different foods. In other 

 words, the rate of senescence differs according to character of food. 

 This suggests the possibility that with certain foods growth might 

 continue and fragmentation be delayed until attainment of the 

 stage of sexual maturity, but only further experiment can throw 

 light upon the question. 



As regards the parasitic groups of flatworms, the flukes and the 

 tapeworms, there can be no doubt that formation of gametes and 

 sexual reproduction is characteristic of an advanced stage in the 

 life of the individual. Such parasites are subjected to but little 

 change in external conditions, especially those living in the bodies 

 of mammals, and yet they pass through a definite life history, ending 

 in the development of gametes and, following this, the death of the 

 individual. In some of the flukes the number of larval generations 

 between the egg and the development of the sexual organs may 

 differ according to external factors, but the relation between 

 sexual maturity and relatively advanced age is unmistakable. 



