1905.] natural sciences of philadelphia. 191 



4. — The Heterochromosomes. 



These were discovered but not correctly interpreted by Henking 

 (1890) and Wilcox (1895); first recognized as modified chromosomes 

 by Paulmier (1899) and myself (1898); then described for a variety of 

 Arthropoda by McCUmg (1899-1904), Sutton (1900, 1902), de Sinety 

 (1901), Wallace (1900), Gross (1904), McGill (1904), Baumgartner 

 (1904), myself (1901a and b, 1904a), Voinov (1903) and Prowazek 

 (1901) ; the last two papers I have not seen. In all these objects there 

 occur in the spermatogenesis peculiarly modified chromosomes, which 

 I have proposed (1904a) to include under the term "heterochromo- 

 somes." I had named them previously "chromatin nucleoli," though 

 with full appreciation of their chromosomal nature, Paulmier " small 

 chromosomes," McClung "accessory chromosomes," and de Sinety 

 "special chromosomes." Their essential characteristic is their differ- 

 ence in behavior from the other chromosomes in the growth period of 

 the spermatocytes and ovocytes, as sometimes during the rest stages 

 of the spermatogonia, a difference which appears usually to consist 

 in the maintenance of their compact structure and deep-staining inten- 

 sity, so that while the other chromosomes become long loops or even 

 compose a reticulum, these do not undergo any such changes or only to 

 slight extent. There is really not much known as yet of these modified 

 chromosomes despite extended studies upon them, and at this place I 

 wish mainly to draw attention to and try to explain differences in their 

 behavior during the matliration mitoses, and so endeavor to explain 

 certain phenomena that up to this time have been regarded as contra- 

 dictory. They appear to be of very general occurrence in insects, have 

 been found in spiders by ]\Iiss Wallace and by me (in the present paper), 

 but so far seem not to be demonstrated for other objects. To be sure 

 Blackman (1900) described an "accessory chromosome" in spermato- 

 cytes of Scolopendra, but did not describe its action in the spermato- 

 gonia nor even in the maturation mitoses, and has not proved in any 

 manner that this body is not a true nucleolus ; true nucleoli containing 

 chromatin or even chromosomes are relatively rare in metazoan cells, 

 but they sometimes occur (as, e.g., I have shown for the ovocyte of 

 Paragordius in a paper recently published), and what Blackman has 

 described appears to be such a structure. 



As I recently pointed out (1904a) there are two main kinds of hetero- 

 chromosomes : such as occur in pairs in the spermatogonia and unite 

 to form bivalent ones in the spermatocytes, which are the most frequent 

 kind in the Hemiptera and were named "chromatin nucleoli" by me; 



