268 The American Naturalist. [April, 



hypothesis of the continuity of unchanged germ cells, and 

 later, when observation in other animals had made this theory 

 untenable, to the theory of the continuity of unchanged germ 

 plasm which is beyond the ken of direct observation. 



If the sex cells are the result of histogenesis, it will be neces- 

 sary to explain their peculiar power. They seem to me to be 

 due to the same processes that have given the retinal cells 

 their peculiar properties. 



Assimilation, reproduction and the closely allied hereditary 

 power are the diagnostic characters of protoplasm. These, 

 with numerous other powers, such as contractility, conductiv- 

 ity and irritability, are the properties of every protozoan cell. 

 Even here we find that certain of these functions are more or 

 less restricted to definite parts of the cell. In the higher ani- 

 mals this differentiation has gone so far that definite functions 

 predominate in highly specialized cells to almost the exclusion 

 of the other powers. 



With this division of labor and the consequent histogenic 

 differentiation of definite cells in the metazoan corm for pur- 

 poses of contraction, conduction and irritation, we have also 

 the differentiation for heredity, and it would be surprising if 

 we did not. 



In lower forms, where the cells of the body often perform 

 many duties, where the division of labor and histogenesis has 

 not been carried to the extreme, many of these cells also retain 

 the hereditary power to a great extent as shown in the power 

 of budding or regeneration. 



There seems to he no necessity to conjure up a substance 

 and processes in the genesis of the reproductive tissues differ- 

 ent from those obtaining in the muscular tissues. 



During the long ages of the rise of animals those j 

 sufficiently differentiated contractile tissue to move the c 

 to food or from danger have survived, and in preciselv the 

 same way those corms containing cells capable of developing 

 into other similar corms have survived. Similar causes have 

 operated in producing each tissue. 



The sex cells are proven to influence the formation of the 

 sex ridge. The peritoneal cells rise to form the ridge only 



