Cytological studies on Salmo irideus

Summary

• 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 bySvärdson.

• 7. The data found inSalmo irideus suggest that chromosome changes according to the Robertson law have to be accepted.