234 Fertilisation and Hybridisation 



taken at the successive stages allow one to follow up, with- 

 out difficulty, the identity of the short and thick nuclear 

 threads. In doing so one sees that, in the double nuclei, 

 the nuclear threads lie in pairs, that is, that there are two 

 nuclear threads of each individual length. Evidently 

 these belong together in such a manner, that in every pair 

 one thread belongs to the paternal and one to the maternal 

 pronucleus. A border line between them is nowhere to be 

 seen, and yet their independence is very evident. And 

 this harmonizes with the conception, as detailed above, 

 that, according to the species examined, this limit can be 

 observed for a longer or shorter time. 



Microscopic examinations teach us, then, to recognize 

 the independence of the two pronuclei, as well as the 

 autonomy of the individual nuclear threads or chromo- 

 somes during the development of the entire body. The 

 agreement of this observation with the phenomena of 

 heredity may be considered as fully established. 



But it is another question whether the individual chro- 

 mosomes correspond also to special groups of hered- 

 itary characters, or, in other words, whether the bearers 

 of the latter are strictly localized in the nuclear threads. 

 Obviously, this question can be answered only physiologi- 

 cally. It amounts to a decision as to whether, if definite 

 chromosomes, or definite parts in them, as for example, 

 single chromomeres and chromioles, were wanting, defi- 

 nite external characters of the organism would also be 

 lacking. If it were possible to kill a nuclear granule with- 

 out otherwise injuring the germ, what would be the con- 

 sequences ? 



Engelmann has taught us, in his revolutionizing in- 

 vestigation on the activity of the individual chlorophyll 

 grains, how the focal point of a lens can be moved over 



