232 Fertilisation and Hybridisation 



whole organism. Their smallness would rather lead us to 

 suppose that every one of them could, at the most, repre- 

 sent only a small group of such units. 



To prove this, on the one hand microscopically, on the 

 other hand experimentally, is the task that Boveri set for 

 himself. 



The filamentous framework in most nuclei, recogniz- 

 able by certain staining methods, is now admitted by all 

 investigators as the idioplasm, the bearer of the hereditary 

 qualities. This thread is very delicate, and seems to form 

 a skein. But when the nucleus prepares to divide, the 

 thread contracts, and thereby is seen, what had hitherto 

 been invisible, that it is composed of several separate 

 threads. In the nucleus there are several threads and not 

 one single one. When the contraction of the thread is ad- 

 vanced so far that the individual parts have become quite 

 short and thick, they are called chromosomes. In the 

 nuclei of the body-cells these always occur in an even 

 number, one-half belonging to the paternal, the other to 

 the maternal pronucleus. 



In a series of classical investigations Boveri succeeded 

 in showing that the individual chromosomes, on elongat- 

 ing again, when the division is accomplished, retain their 

 independence. They remain the same during their whole 

 life, elongating and shortening alternately throughout 

 their entire development. The purpose of the shortening 

 is to make possible an even division of all parts during 

 cell-division; the threads then split lengthwise, in such a 

 way that every single bearer of heredity first doubles, and 

 then sends the two halves into the daughter-nuclei. This, 

 of course, could hardly be accomplished in a skein. On 

 the other hand elongation has for its object the freeing of 

 the bearers of heredity from that crowded accumulation, 



