No. 584] HEREDITY AXD ITS ME AX IX G 



40 1 



nearly all the phyla of the animal kingdom. Sex chro- 

 mosome studies have undoubtedly increased these figures 

 for the animal kingdom to date, by hundreds of species. 



Variation in chromosome number among the cells of an 

 individual plant or animal is a recognized fact among 

 rytologists, but this variation is not regarded as of par- 

 ticular significance, as commonly it is held to exist only 

 among old cells, cells highly specialized, or, at any rate, 

 cells which will never have anything in common with re- 

 production. To quote from Strasburger, 



the number of chromosomes in the nuclei of the somatic cells of both 



far as my experience goes, these observations are always to be observed 

 in the nuclei of cells which are no longer embryonic, like those in an 

 embryo nr growing point, but which, on the contrary, are to some ex- 

 t i'M t histologically specialized arid are not destined eventually to give 

 rise to reproductive cells. The determinate number is still more fre- 

 quently departed from in nuclei which arc definitely excluded from the 



In the reproductive cells, chromosome division is, on 

 the other hand, very exact, and the numbers found, almost 

 invariable, with one exception. This exception is the so- 

 called accessory chromosome or chromosomes, that ap- 

 pear to be coupled with sex differentiation. And the 

 very fact that such accessory chromosomes do exist and 

 by their presence or absence parallel sex distribution, 

 forms one of the most unanswerable arguments in favor 

 of the chromosomes being the chief bearers of character 

 determinants. 



Morphological Individuality of the Chromosomes 

 The next topic to consider is whether there is snlVn-ient 

 evidence to support the idea that these bodies— the chro- 

 mosomes—are morphological entities persisting from one 

 cell generation to another. 



Prochromosomes are deeply staining bodies found in 

 the resting cell nuclei of plants, which probably corre- 

 spond in number, but not in size, to the chromosomes 

 which are found in the dividing nuclei. These bodies are 



