ONTOGENESIS 215 



ess known as fertilization, by which the zygote or ferti- 

 lized cell receives an equal quantity of essential nuclear 

 (chromosome) substance from each gamete, and therefore 

 from each parent of the individual about to be formed. 



It has already been pointed out that the ovum, not 

 being motile, is in a certain sense passive; the sperma- 

 tozoon, which is motile, active in the process. The 

 spermatozoon is no doubt attracted to the ovum by that 

 elementary characteristic of protoplasm already de- 

 scribed as chemotropism. 



The fertilization is differently effected according to 

 the differing conditions of life. Thus, when the animals 

 are aquatic, the ova and spermatozoa may be discharged 

 into the surrounding water and their conjugation trusted 

 to chance. When they are terrestrial, special means 

 must be taken to overcome the obstacles of gravity, 

 etc., and means provided for conveying the spermatozoa 

 to the ova. Many means used by plants for effecting 

 fertilization have already been discussed. Terrestrial 

 animals are usually furnished with sexual organs fitted 

 for coitus so that the sperm of the male may be directly 

 introduced into the organs of the female where fertiliza- 

 tion takes place. 



In plants the external, in animals the internal, morpho- 

 logical characters predominate in importance. This 

 occasions certain fundamental differences in embryology 

 by which the development of the plants becomes a sepa- 

 rate subject, to describe which would, on account of the 

 diversified forms to be considered, divert us from the 

 general scope of this writing. It must, therefore, be 

 left to those intending to pursue botany as a specialty, 

 and be dismissed with the brief statement that it con- 

 forms to the general principle that embryological develop- 

 ment is the passage of the organism from the simplicity 

 of unicellular structure to the complexity of differen- 

 tiated multicellular structure, and that in this trans- 

 formation the embryo passes through a series of stages 

 which suggest the phylogenetic ascent of its kind. The 



