Cytological studies on Salmo irideus.

Abstract

1. In fixing fish eggs to be used in chromosome research Nawashin-Karpechenko was used to advantage. The isolated germ discs were prevented from being damaged by enclosing them in agar. 2. InSalmo irideus the nuclear and cellular divisions proceed entirely synchronously during the first ten or eleven cleavages; phase differences will occur from about the 12th cleavage period, while at the 16th all the phases may occur side by side in one single germ disc. 3. During the successive cleavages the nuclei and the chromosomes very gradually become smaller until a constant size is reached. 4. Owing to the phenomena mentioned under 2 and 3 successful chromosome research is only possible during a short stage. In the case ofSalmo irideus this stage falls between the 11th and the 16th cleavage period. 5. The cleavage rate ofSalmo irideus (Rainbow trout) exceeds that ofSalmo trutta (Brown trout) considerably, viz. 3 hours against 4 hours per cleavage, at 9°C. The reciprocal crosses between these two species have a cleavage rate that is identical to that of the mother species. 6. Salmo irideus possesses 60 chromosomes of which at least 40 chromosomes have median or submedian centromeres. The proportion between the one-armed and the two-armed chromosomes highly deviates from that in the otherSalmonidae investigated bySvardson. 7. The data found inSalmo irideus suggest that chromosome changes according to the Robertson law have to be accepted.