Abstract
Larvae of various stages (5-13mm. in body length and before pupation) in Chironomus dorsalis, were dealt with, and the vesiculation of heavy bands of salivary gland chromosomes, mainly induced by physiological treatment, was studied with the light and phase contrast microscopes (Tiyoda, BM). The treatments were performed prior to aceto-carmine fixation, as follows: (1) glands were exposed for long time (10-12 minutes) to Ringer's solution; (2) treated with N/5 NaCl in weak acid (acetate buffer solution of pH 5); (3) fixed with aceto-carmine in which N/3 Nacl was involved.
The results were discussed on the following three points, in connection with the vesiculation and the chromosomal organization:
1. The vesiculation occurs in each heavy band (band group) in larvae of all stages studied here. This corresponds with the result gotten by Fujii & Kimoto (1955) that the small vesicular bands (bulb formation?) exist in the youngest salivary gland chromosomes (1-3mm. larvae). This would indicate that the heavy band (band group) is already organized in the youngest salivary chromosome.
2. In large chromosomes, a vesiculated heavy band gives two images; one is dispersed chromatin granules with sharp strings in ordinary microscopy, and another one is phase negative, irregular shaped vesicle in phase microscopy. These images must be attributed to different structural elements. The latter image is possibly due to the optical change in chromosomal matrix. This indicates that the latter can not be any evidence for the chromosomal disruption.
3. Finally, considering cytological characteristics of both doublet (Drosophila) and heavy band (Chironomus etc.) i. e., the resistance against stretching, and the ability to vesiculate, it is assumed that they have some common role in chromosomal organization.
