410 INTRODUCTION TO CYTOLOGY 



because of his emphasis upon the need for careful studies of the cell 

 mechanism at the critical stages of the life history, and upon the idea 

 that this mechanism is in some way bound up with the phenomena of 

 heredity. "It has been Weismann's great service to place the keystone 

 between the work of the evolutionists and that of the cytologists, and 

 thus bring the cell-theory and the evolution-theory into organic con- 

 nection" (Wilson 1900, p. 13). 



We may further point out, with Morgan (1915), that the factorial 

 hypothesis assumes only three things about the factors: they are constant, 

 they are usually in duplicate in each body cell and immature germ cell, 

 and they usually segregate in the maturing germ cells. The hypothesis, 

 and the Mendelian theory in general, therefore have to do only with 

 heredity : they do not attempt to explain the causes of development. They 

 seek rather to account for the initial resemblances or differences in here- 

 ditary potentiality which are observed to exist between the germ cells 

 from which successive generations arise. Between the materials com- 

 posing the initial factors and the fully expressed characters of the or- 

 ganism "lies the whole world of embryonic development," to which the 

 application of the theories under consideration has not yet been extended 

 in any systematic or satisfactory manner. Nevertheless many investi- 

 gators, though realizing the failure of Weismann's attempt to explain 

 development in terms of representative particles, are strongly inclined 

 to the view that since the Mendelian characters appearing toward ma- 

 turity behave as though associated with discrete units in the germ, the 

 course of ontogenetic development in its earlier stages must also be due in 

 large part to the activity of factors carried by the nucleus. Development 

 is thus held to be predetermined or controlled by an internal mechanism : 

 external agencies act only by affecting the operation of this mechanism. 

 The factors control the character and behavior of the cells, and upon 

 these in turn the organism, which is a cell aggregate, is alone dependent 

 for its characters and activities. In place of the early hypothesis on 

 which it was supposed that the development of characters is controlled 

 by the migration of determiners or pangens from the nucleus into the 

 cytoplasm at precisely the right times and places, we now have the theory 

 that the factors in the nucleus probably produce their effects by initiating 

 series of chemical reactions which involve all parts of the cell. As Mor- 

 gan (1920) states, " Granting that differences may exist in the nuclei of 

 different species, different end products are expected. The evidence 

 that such differences may be related to specific substances in the nucleus 

 is no longer a speculation but rests on the analytical evidence from Men- 

 delian heredity. In what way and at what times the nuclear materials 

 take part in the determination of characters we do not know. The 

 essential point is that we are in no way committed to any interpretation. 

 Stated negatively we might add that there is nothing known at present 

 to preclude the possibility that the influence is a purely chemical process." 



