THE PHENOMENA OF REVERSION 349 



an account of the whole history of the development of our 

 knowledge of this subject,* as this is unnecessary for under- 

 standing the phenomena of heredity. I must, however, refer 

 to a period of this history, which is instructive as regards the 

 method by which the cell is controlled by the emigration of 

 nuclear matter into the cell-body as enunciated by de Vries and 

 accepted by myself. 



After the so-called • extrusion " of the polar bodies of the egg 

 had been shown to correspond merely to a very unequal cell- 

 division, and convincing proof had been given that the controlling 

 substance must be situated in the chromatin of the nucleus, it 

 followed as a logical postulate that the ovum, like every other 

 cell, must be supposed to contain a special controlling substance, 

 or specific idioplasm^ the function of which is to produce the spe- 

 cial histological nature of the cell in question. This conclusion 

 I arrived at, and assumed that the ovum, from its earliest stage 

 to the attainment of its full size and specific nature, is controlled 

 by a special idioplasm, which differs entirely from the idio- 

 plasm which becomes active after the completion of maturation. 

 If the nature of the cell is determined at all by its idioplasm, 

 the ovum, while still growing and undergoing histological de- 

 velopment, cannot possibly be controlled by the same idioplasm 

 as that which serves for embryonic development. I consequently 

 assumed the existence of an ^oogenetic' idioplasm in the egg 

 during the period of its histological differentiation, and also that 

 after maturation, this substance gives up the control of the cell 

 to the germ-plasm. 



The question then arises as to what becomes of the oogenetic 

 idioplasm when this change in the control takes place. 



My answer to this question was, that the oogenetic idioplasm 

 is removed from the ovum by means of the polar divisions, and 

 that it was thus rendered possible for the germ-plasm — which 

 was already present in the nucleus of the ovum, and had in the 

 meantime increased considerably in bulk — to obtain control of 

 the cell. 



This conjecture has since turned out to be erroneous. Investi- 

 gations which I subsequently made soon showed that at least 

 one of the two polar divisions has a totally different significance, 



* Such an account is given in my essay on 'Amphimixis,' and in still 

 greater detail in the previous ' Essays upon Heredity.' 



