CONDITIONS OF GAMETE FORMATION 389 



manders, for example, may be made to undergo metamorphosis 

 into the adult form at a very small size or to attain giantism without 

 metamorphosis by controlling the character of the food (Guder- 

 natsch, '12, '14; Romeis, '14), and their development may be modi- 

 fied in various other ways. Even in man the age in years at which 

 sexual maturity occurs varies somewhat widely with climatic and 

 other factors. But none of these facts indicate anything more than 

 that a certain physiological condition of the organism may be 

 attained sooner or later according to the nature of the environment. 



In various species among both invertebrates and vertebrates 

 cases of premature sexual maturity may occur while the animal is 

 still morphologically a larva, as in the so-called axolotl form of 

 certain salamanders; or after sexual maturity in the larval stage 

 the sex organs may disappear and the animal undergo meta- 

 morphosis to the adult form, after which new sex organs arise 

 and a second period of sexual maturity occurs, as in certain 

 ctenophores. 



Evidently the sex organs may mature and produce gametes 

 at various stages of morphological development, but we know 

 nothing of the physiological conditions in these cases. In the light 

 of the facts already cited, however, it is probable that, whatever 

 the morphological stage at which sexual maturity occurs, certain 

 physiological conditions must exist in the organism which make its 

 appearance possible and that these are conditions which ordinarily 

 arise relatively late in development. In other words, the cases 

 of premature sexual maturity are probably cases of accelerated 

 physiological senescence. 



PARTHENOGENESIS AND ZYGOGENESIS 



In several of the invertebrate groups, viz., the rotifers, the clado- 

 cera among the Crustacea, and the plant lice and related families 

 among the insects, the eggs of a single individual or of successive 

 generations differ in behavior, some developing parthenogenically 

 into females or males, and others zygogenically, i.e., requiring 

 fertihzation for development. 



Within recent years members of the crustacean group of 

 cladocera, the daphnids, have been the subject of extensive 



