ESSENTIAL FEATURES OF SEXUAL PROCESS 1345 



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 

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



FIG. 552. Origin of the primordial germ-Cells and casting out of chromatin in 

 the somatic cells of Ascaris. ( WILSON after BOVBRI.) 



A, two-cell stage dividing ; s, stem-cell, from which arise the germ-cells. 

 - B, the same from^the side, later in the second cleavage, showing the two types 

 of mitosis and the casting out of chromatin (c) in the somatic cell, c, resulting 

 four-cell stage ; the eliminated ohromatin at c. D, the third cleavage, repeat- 

 ing the foregoing process in the two upper cells. 



of independent existence or of forming the divergent series of types 

 necessary for the formation 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 capable of developing 

 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 pro- 

 genitor of the germ-cells differs from the somatic cells both by the 

 greater size of its nucleus and in its mode of division (Fig. 552). In the 



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