Heteropycnosis 



487 



called heterochromosomes to differentiate them from the auto- 

 somes, which were then called euchromosomes. 



The particular stage of the life cycle at which the sex chromo- 

 somes show heteropycnosis appears to vary in different animals, 

 and in some it does not appear to occur at all. It sometimes is 

 found after the first spermatogonial division in animals in which 

 the male is the heterogametic sex, and less frequently is found 

 in the spermatogonia. In some organisms the heteropycnotic 



Fig. 139. Heteropycnotic chromosome A of Phrynotettix : (a) diplotene; 

 (b) late prophase. (Courtesy of Dr. D. H. Wem-ich in Bulletin oj the 

 Museum of Comparative Anatomy of Harvard University.) 



condition is observed in the prophase stages of the first meiotic 

 division, or division of the primary spermatocyte, and it is fre- 

 quently seen in the interkinetic stage between the two spermato- 

 cyte divisions. It is also found in the spermatids for a consid- 

 erable time after the second meiotic division. 



Heteropycnosis has recently been used by S. G. Smith to dif- 

 ferentiate the two sexes during early stages of development in 

 the spruce budworm, Archips jumijerana. It was desired to de- 

 termine the sex ratio before high mortality occurs during larval 

 development to learn whether this mortality aft'ects the two 

 sexes differently. Fortunately, the sex chromosome remains 

 heteropycnotic in the resting cells of the female of this animal 

 in many different kinds of somatic tissue, and especially in the 

 large cells of the silk glands. Since the female is heterogametic 

 in this organism, the heteropycnotic chromosome is found in that 

 sex (Fig. 140). By examining the silk glands for cells with heter- 

 opycnotic chromosomes the sex can be determined during an 

 early stage of development. 



