142 SYSTEMATIC ZOOLOGY. 



zoan, unites with the egg, and then the compound cell is 

 capable of development.* Reduced to its simplest terms, 

 the process of development may be briefly stated thus: 



After union with the sperm-cell (fertilization) the egg 

 divides again and again, the result being the formation 

 of a large number of cells, all connected together, which 

 later arrange themselves in layers (p. 154) and then 

 develop into organs. This type of reproduction is known 

 as sexual reproduction, since egg-cell and sperm-cell are 

 produced by animals of different sexes. 



In many Protozoa something similar occurs. Here we 

 find a union of different individuals, and as each protozoan 

 is a single cell, this union of individuals is comparable, to 

 a certain extent, to the union of egg-cell and sperm-cell. 

 With most Protozoa, however, after this union (conjuga- 

 tion) the individuals separate and each divides, thus pro- 

 ducing new individuals (cells), which differ from the cells 

 produced by the division of the eggs in that they never 

 arrange themselves into layers, but each forms a dis- 

 tinct individual like the parent. 



CLASSIFICATION. 



In order to show the relationships of the various forms 

 of organized life resort is had to classification in which 

 forms very near each other are grouped together, and 

 these groups in turn are similarly united, and so on until 

 the whole living world is included in the largest group. 

 The process and the principles involved can best be illus- 

 trated by a concrete case. 



* In a few cases, as in the honey-bee, the eggs can normally 

 develop without union with a spermatozoan. This is called par- 

 thenogenesis. 



