1254 PHYSIOLOGY 



by bd-\ ac or bd may be better or worse combinations than ab or cd. If 

 either of them is better, that organism will survive under the less favourable 

 conditions, and the race will continue with a slight, and to us inappreciable, 

 change of type. 



REPRODUCTION IN THE METAZOA 



The numberless cells forming the bodies of the higher animals are all 

 produced by a series of divisions from a single cell, the fertilised ovum. 

 This cell is the result of a process of conjugation between two cells derived 

 from different individuals. With the multiplication of cells forming a siu^l.- 

 organism there is, of course, an increased size of the organism. It is doubtful 

 whether this of itself would be of any advantage, were it not that the 

 multiplication of cells goes hand in hand with differentiation, groups of 

 cells being modified structurally and set aside for one or other function of the 

 body. Differentiation of function implies higher functional capacity. 

 As a motor organ or as a means of locomotion, the differentiated muscle 

 cells, with their attached parts, must be more effective than the undif- 

 ferentiated protoplasm of the amoeba. Specialisation of function involves 

 changes of type in the cells resulting from the division of the primitive un- 

 differentiated ovum. In most cases this change of type is permanent. An 

 epithelial cell such as that forming the epidermis or the liver, when it divides, 

 produces another cell of the same kind. One might almost speak of the 

 evolution of a new species of cell, but that it takes place within the short 

 period of the development of the multicellular individual, instead of occupy- 

 ing a long space of time and involving the destruction of countless indi- 

 viduals, as is the case when a change of type gradually occurs in unicellular 

 organisms. Differentiation necessarily brings with it a limitation of the 

 powers of reproduction. Any one of the descendants of a unicellular 

 organism is in all respects equivalent to its ancestor, and can reproduce the 

 same type of individual. The specialised liver or muscle cell can produce 

 only a cell of the same type, one, that is to say, incapable of independent 

 existence or of forming the divergent series of types necessary for the pro- 

 duction of an individual. Differentiation of function therefore involves the 

 setting aside of certain cells, germ cells, which retain their primitive character 

 and are capable of indefinite division to form new generations each" able 

 to develop into a complete individual. These germ cells can often be 

 recognised from the very earliest divisions of the fertilised ovum, which lead 

 to the production of the mature individual. Thus in Ascaris, the progenitor 

 of the germ cells differs from the somatic cells both by the greater size of 

 its nucleus and in its mode of division (Fig. 567). In the cells destined to 

 produce the somatic cells, a portion of the chromatin is cast out into the 

 cytoplasm, where it degenerates, so that only in the germ cells is the sum 

 total of the chromatin retained. Thus in the two-celled stage, in one cell 

 all the chromatin is preserved, while in the other cell the thickened ends of 

 the chromosomes are cast off into the cytoplasm and degenerate, only the 

 thinner central portions being preserved. When these divide again, the 



