SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 261 



A moment's consideration, however, will show that in most 

 cases the organism does not wholly die. Some of the cells 

 usually escape from the bondage of the body as reproductive 

 elements, — as, in fact, Protozoa once more. The majority of 

 these may indeed be lost ; eggs which do not meet with male 

 elements perish, and the latter have even less power of inde- 

 pendent vitality. But when the ova are fertilised, and proceed 

 to develop into other individuals, it is plain that the parent 

 organisms have not wholly died, since two of their cells have 

 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 fertilised 

 ovum. This divides, and its daughter-cells divide and redivide. 



The relation between reproductive cells 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 fertilising the 

 liberated ovum is also indicated. 



They arrange themselves in layers, and are gradually mapped 

 out into the various tissues or organs. In division of labour, 

 they become restricted in their functions, and specialised 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 recuperation, and emphatically liable to local 

 and periodic, or to general 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 differentiation 



