ONTOGENESIS 197 



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 

 significance of this expression will be better understood 

 after the perusal of the matter that is to follow. 



Every metazoan begins its life history as a single cell 

 or egg, and whether this is a distinctly differentiated 

 germinal cell or egg or an indistinctly differentiated 

 germinal cell such as forms the starting point of the 

 gemmation of ccelenterates, etc., makes no essential 

 difference. 



For the present, however, we shall neglect the undif- 

 ferentiated and consider only the differentiated germinal 

 cells the true eggs. 



These present a great variety of appearances, but 

 little difference of structure, as each is a single cell. They 



