220 REPRODUCTION 



they are anisogamous and thus show the beginnings of sex. The 

 resemblance between the processes of sexual reproduction in pro- 

 tozoa and metazoa is, therefore, as obvious as the homology 

 between cells that exists throughout the Animal Kingdom. For 

 this reason the term gametes is applied to the conjugating cells 

 of protozoa and to the ova and spermatozoa of metazoa; and the 

 term syngamy is used to include both conjugation and fertiUza- 

 tion. If anything be needed to complete the resemblance, the 

 existence in some protozoa of processes resembling the maturation 

 of gametes in metazoa presents conclusive evidence of the funda- 

 mental similarity of these sexual processes in all forms of animal 

 life. As to the functional significance of conjugation and fer- 

 tilization, it is clear that each furnishes a basis for biparental 

 inheritance; but whether conjugation and fertilization have 

 other features in common is uncertain (c/. p. 187). 



The syngamic union of a male and a female cell to form a single 

 cell, the zygote, which is a new individual, is, therefore, the essen- 

 tial feature of sexual reproduction. In the protozoan this individ- 

 ual remains an isolated cell. In the metazoan it develops into a 

 many-celled organism by cell division and differentiation. As this 

 development proceeds, the biparental nature of the original cell 

 supposedly persists in all the cells of the individual; and in this 

 manner the biparental inheritance that appears in an adult is 

 based upon the bisexual origin of every cell of its body when traced 

 back to the one-cell stage. Thus, sexual reproduction and devel- 

 opment are inextricably related ; but the comparison with protozoa 

 is clarified if sexual reproduction in the metazoa is thus defined 

 as the origin of a new individual by syngamy and distinguished 

 from the development that follows. 



Modifications of sexual reproduction appear as parthenogenesis, 

 both natural and artificial (p. 235); and as pcedoge7iesis, in 

 which the organism becomes sexually mature in a larval stage in 

 particular generations and may never reach the fully developed 

 adult condition. Examples of pedogenesis are seen in the axolotl, 

 an amphibian which reproduces sexually in a stage comparable 

 with the late tadpole stages of other salamanders; in the liver 

 fluke, which reproduces parthenogenetically through a series of 

 larval generations; and in some insects. 



