IO C. M. CHILD. 



except for nutrition of the soma or are the gametes physiologically 

 integral parts of the individual? Second, do the gametes before 

 fertilization actually consist of undifferentiated germ plasm or 

 are they differentiated cells like other parts of the organism: in 

 other words are the gametes physiologically young or old cells? 

 Third, why does the egg usually require the special stimulus of 

 fertilization or some artificial stimulus in place of it for the 

 initiation of development? Fourth, why are some eggs 

 naturally parthenogenetic and what is the nature of artificial 

 parthenogenesis? Fifth, how shall we interpret the alternation 

 of generations in plants and metagenesis in animals? Sixth, what 

 is the relation between asexual and sexual reproduction and why 

 is sexual reproduction the only method of reproduction in the 

 higher animals? We shall consider these questions in order. 



i . The Origin and Formation of the Gametes. 



The fact that the primitive germ cells can be distinguished very 

 early in development in certain animals, e. g., Ascaris, Cyclops 

 and various vertebrates, is familiar to all and has commonly been 

 regarded as strong evidence in support of the germ plasm hypothe- 

 sis. In those cases in which the germ cells do not appear until 

 the adult stage is reached it is of course easy to assume that the 

 germ cells are there throughout development, but are simply not 

 visibly different from other cells during the earlier stages. But 

 is this interpretation the most satisfactory? Does it account 

 for the facts in the simplest way? I believe that it does not. 



In the first place all forms in which the germ cells are dis- 

 tinguishable in early cleavage or embryonic stages are forms in 

 which the specifications of the embryonic cells become fixed 

 at an early stage. The germ cells, like other organs, become dis- 

 tinguishable early in the developmental history. The case of 

 Ascaris, however, stands to some extent apart from the others, 

 for here the first cleavage separates the egg into two cells, one 

 of which represents the germ cells plus considerable parts of the 

 soma, while the other represents somatic parts alone and under- 

 goes the process known as diminution. The undiminished nu- 

 cleus is commonly supposed to represent the germ plasm and in 

 this case the hypothesis of continuity seems to be confirmed by 

 observation. 



