REPRODUCTION 209 



occurs. Related phenomena also make their appearance 

 among unicellular organisms. 



Though insufficient data are at hand to enable accurate 

 generalization to be made, it seems safe to assume that 

 in such plants and animals as are subject to parthenoge- 

 netic development, or development' from unfertilized eggs 

 or germinal cells, reduction of chromosomes does not occur. 



Many interesting examples of the special means by 

 which reduction of chromosomes is effected among 

 plants might be given, but at an expense of space that 

 would scarcely be worth while in a writing not particu- 

 larly devoted to plant physiology. In dismissing the 

 subject, however, one fact should be mentioned, that is, 

 that among the ferns, mosses, and algae, where alterna- 

 tion of generations exists, the asexual generations 

 (sporophytes) are diploid i.e., have spores possessing 

 the somatic number of chromosomes, while the sexual 

 generations (gametophytes) are haploid i.e., produce 

 sexual cells, or gametes, having the reduced number; 

 and also that among animals, such as certain nematode 

 worms angiostomum in which hermaphroditic genera- 

 tions alternate with bisexual generations, the hermaphro- 

 ditic individuals have the diploid, and the bisexual gen- 

 erations the haploid number of chromosomes. 



The fertilized cell, or zygote, is immediately ready for 

 development into the new individual, which, through 

 the receipt of an equal number of chromosomes from 

 each parent, inherits characteristics from each. The 

 development of the zygote into the new individual forms 

 a new phase for study, known as ontogenesis. 



Before proceeding, however, it may be well to inquire 

 whether, in the present state of knowledge, we are jus- 

 tified in attributing to the chromosomes of the male 

 and female pronuclei the source of parental and ma- 

 ternal characters. This subject will be considered at 

 some length in a future chapter, but it seems wise at 

 present to say a word or two concerning the evidence. 

 Boveri has succeeded in rearing an echinoderm larva, 

 14 



