2 INTRODUCTION. 



In the higher Metazoa differentiation is carried much further, 

 the number of kinds of cells is greatly increased, and the dif- 

 ferences between them are far more pronounced; so much so, 

 indeed, that in a single organ, as the heart of a rabbit, there may 

 be more kinds of cells, and cells differing more widely from one 

 another both in structure and in function, than are to be found 

 in the entire body of one of the simpler Metazoa, such as a hydra. 



In all Metazoa, however, whether high or low in the 

 scale of organisation, a distinction may be drawn between 

 the cells which compose the body of the individual proper, and 

 certain other cells which are concerned, not with the welfare 

 of the individual animal, but with the perpetuation of the 

 species. An adult Metazoon consists, in fact, of two chief kinds 

 of cells, somatic and reproductive, of which the former build up 

 the various tissues and organs of the animal itself; while the 

 latter, i.e. the eggs or germ-cells of the female, and the sperm- 

 cells of the male, contribute nothing towards the maintenance 

 of the animal itself, but provide for the production in due time 

 of future generations of similar animals. 



All Metazoa reproduce by means of eggs ; and these eggs 

 are in all cases component cells of the animals in which they 

 occur. Other modes of reproduction are seen, especially in the 

 lower Metazoa, such as the budding of a sponge or of a hydra, 

 but these only alternate with sexual reproduction, egg-producing 

 individuals always occurring sooner or later in the series. 



The distinction between Protozoa and Metazoa may now be 

 stated more fully. Protozoa are animals which begin their 

 existence as single cells, and which remain single cells through- 

 out their whole lives. Metazoa are animals which begin their 

 existence as eggs, i.e. which commence, like Protozoa, as 

 single cells ; but in the course of development become multi- 

 cellular, the majority of the constituent cells becoming modified 

 to form the various parts of the adult animal, while some become 

 reproductive cells, which contribute nothing towards the wel- 

 fare of the animal itself, but which provide for the continuance 

 of the species. 



The life history of a Metazoon usually shows a more or less 

 marked division into two periods or stages, nutritive and repro- 

 ductive ; the growth of the individual being completed, or nearly 

 so, before the reproductive phase commences. In Vertebrates, 



