THE GF.R^r-PLASM 7 I 



and multiply without interruption during this period, as may be 

 deduced from the fact that the amount of the nuclear substance 

 in the individual cell does not decrease during embryogeny. 

 although such an enormous increase in the number of cells 

 takes place. No accurate and methodical observations have at 

 present been made with regard to the comparative size of the 

 chromosomes in the various stages of development and in the 

 different organs of the body, but it may nevertheless be taken as 

 certain that the entire mass of the nuclear substance grows con- 

 siderably during embryonic development. It appears to me, 

 however, to follow from the observations of Ruckert 1 have 

 already referred to concerning the chromosomes of the ovum of 

 the dog-fish,* that the most marked growth of the determinants 

 takes place immediately before, and during, their activity. Dur- 

 ing the period of growth and histological differentiation of the 

 egg in this fish the idants grow enormouslv, and towards the 

 completion of these processes they gradually decrease in size, 

 until finally, when the ovum is ripe, they have become almost as 

 small as they were originally. 



This may be expressed, in the terms we have adopted, as 

 follows : the determinants which control the histological struc- 

 ture of the egg\ multiply enormously during the growth of the 

 ovum, into the body of which they transmit their 7iumeroits 

 biophors. After this has occurred, only those determinants of 

 the germ-plasm are left which have in the meantime been in- 

 active, and which have only increased to a slight extent ; these 

 are thus contained in those idants which are not much larger 

 than they were in the young egg-cell. From the beginning of 

 ontogeny and onwards, one determinant after another becomes 

 active, and during their activity they also multiply. It has for 

 a long time appeared to me probable that the determination of 

 a cell does not take place, as one might suppose, by the agency 

 of a single determinant, but bv that of many determinants of 



* Anat. Anzeiger, loth March 1892. 



t These determinants of the ovum correspond to the ' oogenetic nucleo- 

 plasm ' of my earlier essays, and constitute the substance which deter- 

 mines the growth and histological differentiation of the egg. For a long 

 time I believed that this substance was extruded from the ovum at the 

 close of the period of maturation by means of the polar bodies. We now 

 see that such an extrusion is not required, as this substance is used up in 

 the differentiation of the tg'g. 



