REPRODUCTION 197 



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. 



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, 

 exhibiting only paternal characters from the enucleated 

 egg of one species fertilized by the sperm of another. 

 This seems to be conclusive, but it is apparently offset 

 by an experiment of Kupelweiser and Loeb, who obtained 

 a larva showing no paternal characters at all from the 

 ovum of a sea-urchin, fertilized by the spermatozoon of 

 a mollusc. However, in the latter case we can be fairly 

 sure that the egg was induced to develop partheno- 

 genetically through contact with stimulating substances 

 contained in the molluscan spermatozoon, and that no 

 amphimixis took place, as the heterologous sperm cells 

 always die in an ovum of such distant relationship, and 

 the paternal chromosomes are lost in consequence. 



In order that ova shall develop they must ordinarily 

 be fertilized, but this is not in all cases essential. J. 

 Loeb has done much to convince us that the stimuli that 

 lead to development are chemical or mechanical, and has 



