PROTOZOA. 1 1 



f an y gi ven species has been considered a more or 

 less trustworthy guide for determining the phylogeny, or 

 historic evolution, of the phylon or tribe to which the 

 animal that produced the egg belonged. This egg, or 

 ovum, as generally described, is a cell made of protoplasm 

 and containing a nucleus within which is a nucleolus. 

 It is, however, reasonable to suppose that this nucleated 

 and therefore differentiated condition of protoplasm arose 

 from an unnucleated and undifferentiated condition, and, 

 therefore, we seek for what may be called the ancestral 

 stages of the nucleated egg. If these stages have become 

 obliterated in the eggs of the more specialized animals by 

 the action of the law of acceleration in development, it 

 would seem probable that they might be represented by 

 the unnucleated adults of the Protozoa, and that a study 

 of these simplest, one celled organisms, and of the spe- 

 cializations leading from an unnucleated to a nucleated 

 condition of protoplasm, might throw light not only on 

 the origin of the nucleated egg, but also upon the natural 

 classification of the Protozoa. Thirty years ago it would 

 not have been possible to attempt such a classification of 

 this subkingdom. But the observations and experiments 

 of many investigators during the past few years have 

 thrown strong light upon the structure and development 

 of many species and the possible phylogenetic history of 

 several groups. The elaborate work of Biitschli, com- 

 prising three volumes of Bronn's Thier-Reich, and the 

 great work of Haeckel, the Challenger Report on the 

 Radiolaria, have both been published since 1880. 

 Besides these, a large number of original papers and sev- 

 eral special works on different groups have appeared 

 since 1874; notable among these are the writings of Gru- 

 ber, Hertwigand Lesser, Cienkowski, Schultze, Grenacher, 

 Brandt, Verworn, Maupas, Mereschkowsky, Plate, Hofer, 

 and of Leidy, W. Saville Kent, Brady, Dallinger and 

 Drysdale, Lang, E. Ray Lankester, Hyatt, Ryder, Archer, 

 Bessels, Calkins, Wilson, and others. Although we have 



