PHYSIOLOGY OF SEX AND REPRODUCTION. 241 



united to start afresh as new plants or animals. In other words, what 

 is new in the multicellular organism, namely, the " body," does indeed 

 die, but the reproductive elements, which correspond to the Protozoa, 

 live on. 



This may be made more definite in the following diagram. There 

 it is seen that the organism starts like a protozoon, as a single cell, or 

 usually as a union of two cells in the fertilized ovum. This divides, 

 and its daughter- cells divide and redivide. They arrange themselves 

 in layers, and are gradually mapped out into the various tissues or 

 organs. In division of labor, they become restricted in their functions, 

 and specialized in their structure. They become differentiated as 

 muscle-cells, nerve-cells, gland-cells, and so on. The result is a more 

 or less complex " body," unstable in its equilibrium because of its very 

 complexity, composed moreover of competing cells far removed from 

 the protozoon all-roundness of function, limited in their powers of 



Fig. 95. — The relation between reproductive ceils and the body The continuous chain of dotted cells 

 at first represents a succession of Protozoa; further on, it represents the ova from which 

 the "bodies" (undotted) are produced. At each generation, a spermatozoon fertilizing 

 the liberated ovum is also indicated. 



recuperation, and emphatically liable to local and periodic, or to gen- 

 eral and final death. But the body is not all. At an early stage in 

 some cases, sooner or later always, reproductive cells are set apart. 

 These remain simple and undifferentiated, preserving the structural 

 and functional traditions of the original germ-cell. These cells, and 

 the results of their division, are but little implicated in the differen- 

 tiation which makes the multicellular organism what it is; they remain 

 simple primitive cells like the Protozoa, and in a sense they too share 

 the protozoon immortality. The diagram shows how one of these 

 cells, separated from the parent organism (and uniting in most cases 

 with a germ-cell of different origin) becomes the beginning of a new 

 body, and, at the same time, necessarily the origin of a new chain, or 

 rather of a continued chain of fresh reproductive cells. 



" The body or soma,"' Weismann says," thus appears to a certain 

 extent as a subsidiary appendage of the true bearers of the life, — the 

 reproductive cells." Ray Lankester has again well expressed this: — 

 "Among the multicellular animals, certain cells are separated from the 

 rest of the constituent units of the body, as egg-cells and sperm-cells; 



