The Life-Cycle of Cladocera. 



433 



insight into the physiological basis of the antagonism between growth and 

 sexual maturity which undoubtedly exists in the Crustacea, and the principle 

 applies with modifications to organisms in general. This antagonism is seen 

 to be due to the necessity for the mature organism to produce a special kind 

 of nutriment for the reproductive organs, so that there is a corresponding 

 lack of the suitable reserve substances for the purposes of growth. In the 

 Crustacea and at some phase of the reproductive period in all organisms, the 

 elaboration of fat for the supply of the ovary or accessory organs of repro- 

 duction is a marked feature of the metabolism in the mature female, and the 

 diversion of reserve material in this form and for this purpose inhibits 

 growth. In the male it is less obvious in what special form the reserve 

 material for tlie nourishment of the reproductive organs is prepared, but 

 here, again, it is probable that fat plays an important part, though the 

 manner of its utilisation is certainly different from the comparatively passive 

 transference which occurs in the female. The alteration in the metabolism 

 thus brought about at sexual maturity, differing in its mode of operation in 

 the male and female, we hold to be responsible for those morphological and 

 physiological changes in the body which often accompany sexual maturity 

 and are known as correlated secondary sexual characters. 



The view developed here as to the nature of sexual maturity and its 

 antagonism to growth has an interesting bearing on the meaning of sex in 

 general. Speaking broadly, the onset of the sexual mode of reproduction in 

 organisms occurs under conditions when continued growth or asexual 

 multiplication is hindered either by lack of appropriate food or accumulation 

 of excretory matter or by some internal weakening of the assimilative 

 capacity. Under such conditions the organism responds by laying up 

 reserve material for a special kind of resting reproductive cell instead of 

 continuing to expend it in growth. The sexual mode of reproduction is thus 

 a means of lying dormant during conditions unfavourable to continued 

 growth. The differentiation into male and female may be looked upon as an 

 economy or division of labour by which the female reproductive cell stores 

 up compactly a mass of reserve material to be used for the nourishment of 

 the next generation, but thereby loses the power of division, whiie the male 

 reproductive cell retains the kinetic energy for division but relies on the 

 female cell to supply the material for development. 



Summary. 



1. By isolating the young Daphnia at birth and keeping them at 27° C. it 

 has been possible to breed them for 19 generations without the appearance of 

 males or ephippial females,3752 parthenogenetic females having been produced. 



