378 RECORDS. 



great, /. c, that the prelocaHzation is of a somewhat general 

 character. This question appears, however, to be of relatively 

 minor importance in view of an additional conclusion given by 

 detailed studies on the formation, maturation and early develop- 

 ment of the ^g%. These studies leave no doubt that the 

 grouping of materials observed at the time the ^gg begins its 

 process of division is not, in some cases at least, a primary or 

 original one, but is of secondary origin. They indicate further 

 that early in the development the ^gg contains only a few of 

 these specific stuffs, at the very beginning possibly none, and 

 that as development goes forward new stuffs are progressively 

 formed and distributed. Now, if this conclusion is well founded, 

 the actual progressive development of the protoplasm must be 

 conceived as a process of cpigencsis, not of preformation and 

 evolution. This is the first general result that I desire to 

 emphasize ; and it is in harmony with the fact, on which all 

 embryologists have been agreed, since the time of Wolff, that in 

 its obvious features development is by the formation and addition 

 of new parts not previously existent as such in the ^%%- The 

 embryo is not actually preformed or even predelineated in the 

 protoplasm from the beginning. The protoplasmic stuffs appear 

 to be only the immediate means or efficient causes of differentia- 

 tion ; and we have still to seek its primary determination in 

 causes that lie more deeply. We are thus led to a brief con- 

 sideration of the question of the physical basis of heredity, 

 which will direct our attention to an element that has hitherto 

 been disregarded, namely, the nucleus, and bring us to a second 

 general result. 



It was long since suggested by Nageli that there is a particu- 

 lar substance or "idioplasm" peculiar to each species of plant 

 or animal that is transmitted in the germ -cells and has the power 

 to determine the development of the ^g'g according to its nature. 

 Later research has given very strong reason to accept this view 

 in principle, and for the further conclusion that this physical 

 basis is represented by a substance contained within the nucleus 

 and known to cytologists as "chromatin." Pas.sing over the 

 cogent, and I believe steadily accumulating, evidence on which 



