156 ROBERT T, HANCE 



VII. Discussion 184 



The work of other mammalian cytologists 184 



Reported cases of fragmentation 190 



The metrical study of chromosomes 192 



Chromosomes in development 193 



The individuality of the chromosomes 196 



VIII. Summary 196 



IX. Bibliography 199 



INTRODUCTIOX 



The terms spermatogonia! and somatic have been frequently 

 used synonymously, with respect to the diploid chromosome num- 

 ber, on evidence considerably more theoretical than direct. As 

 a matter of fact comparatively little work has been done on the 

 body cells, as the germ cells, because of their genetic signifi- 

 cance, have, until recently, largely held the interest of cytolo- 

 gists. Important as is the theoretical significance of the chromo- 

 somes of the germ cells, it seems to me that the chromosomes 

 of the body are fully as interesting. In the soma, as the various 

 cells differentiate in to the tissues that constitute the mature 

 plant or animal, we can see in operation the chromatin whose 

 maturation phenomena we trace through the reproductive cells. 

 What, we may ask, is the behavior of the chromosomes in de- 

 velopment? Are they immutable, or do they undergo change? 

 If changes occur, are they correlated with the character of the 

 tissue in which the variation is found? Also in the event of 

 change what bearing does it have on the theory of the individu- 

 ality of chromosomes? 



Four years ago I selected the pig for studying the problems 

 suggested above. The methods of fixation, then in common use, 

 proved hopeless, as the chromosomes, following the use of the 

 usual fixatives, were clumped together beyond hope of recogni- 

 tion — a condition quite typical of the majority of published 

 figures of mammalian mitoses. My work was consequently de- 

 layed until a method was discovered which preserves the chromo- 

 somes in a condition comparable to the best cytological prepara- 

 tions. The results obtained with this technique (Hance, '17 b) 

 are considered to be representative of the living structures as 



