Prochromosomes 223 



chromatic and the other has just a small heterochromatic region 

 at the centromere. Apparently the autosomes have no hetero- 

 chromatin or just a very small section near the centromere. 



In D. pseudoobscura, the X chromosome also contains two 

 arms which unite to form a V-shaped structure. One of these 

 arms has a large amount of heterochromatin, but the other arm 

 of the X chromosome and two autosomes have considerably less. 

 The third autosome and the small autosome corresponding to 

 chromosome IV of D. melanogaster are mostly euchromatic. A 

 still different situation is found in D. funebris, which has a long 

 rod-shaped X chromosome, about half of which is heterochro- 

 matic, and four pairs of rod-shaped autosomes w^hich have very 

 small regions of heterochromatin near the centromeres. 



In D. pallidipennis, a recently discovered species from Brazil, 

 Dobzhansky showed that all the heterochromatin is concentrated 

 in the X and Y chromosomes except for a few interstitial sec- 

 tions in the autosomes. In this species, the X and Y chromo- 

 somes are enormous and consist chiefly of heterochromatin. The 

 centromere ends of the autosome have little heterochromatin, if 

 any, and all the autosomes are only occasionally connected with 

 the chromocenter in the salivary gland nuclei. Frequently there 

 is an association of two, three, or four of the autosomes by their 

 bases, and one, two, three, or four autosomes may be in contact 

 with the heterochromatin of the X chromosome by their bases. 

 In the X chromosome there is no sharp boundary between the 

 heterochromatin and the euchromatin, and many discs appear 

 to be heterochromatic in some cells and of euchromatin in others. 



Prochromosomes 



In the resting nuclei of many organisms, a number of deeply 

 staining bodies have been observed which frequently correspond 

 to the number of chromosomes seen during mitosis. They ap- 

 pear to be segments of chromosomes that have remained con- 

 densed and must have a high nucleic acid content. Various terms 

 have been applied to them, but those most commonly used are 

 prochromosomes and chromocentric regions. They are probably 

 heterochromatic regions of chromosomes. That they exist fre- 

 quently in the same number as the chromosomes would indicate 

 that each chromosome in many species has just one large hetero- 



