192 ROBERT T. HANCE 



although 58, 61 and 62 chromosomes have been found in the 

 soma. This has been brought about by the fragmentation of 

 the X and Y chromosomes. It is confined to these bodies, and 

 Seiler points out that the breaking up of the sex chromosomes in 

 somatic nuclei has been observed quite often. 



The extra number of chromosomes found in giant cells is not 

 due to the same causes as the above, and they are found in the 

 germ tissue as well as in the soma. I have shown the chromo- 

 some relation of a giant cell to the normal cells. 



The work of Delia Valle ('09) and his conclusions in regard to 

 the variability of chromosomes, is so obviously faulty that I 

 shall not consider them here, particularly since Dr. McClung 

 has discussed his observations and theories at length in a recent 

 paper ('17). 



From the above synopses of reported cases of chromosome 

 variations it is evident that the somatic number has been dis- 

 covered to be different from the spermatogonial number in a 

 few cases and that this is largely due to the breaking up of the 

 major or, what I have termed, the 'type' chromosomes, and in 

 certain cases the same chromosomes are always affected while 

 the others remain intact. If the various workers had measured 

 the chromosomes in the tissues studied we should have more 

 information in regard to the constancy of the chromatin elements. 



The 7netrical study of chromosomes 



Not very much work has been done in the way of measuring 

 chromosomes, and some of the studies along this line have not 

 produced results of value. Among these I think we may place 

 the work of Meek ('12 a, '12 b and '15), whose work was ex- 

 ceedingly carefully executed, but whose conclusions had to be 

 retracted eventually as they were too sweeping and based on 

 insufficient data. 



Farmer and Digby ('14) found that the "total amount of 

 chromatin substance in the nucleus of each of two types of hy- 

 brids known as Primula kewensis is the same. The nuclei of 

 the one form of hybrid contains twice as many chromosomes as 



