FURTHER EXPERIMENTS IN CROSS- AND SELF-FERTILIZATION OF CIONA AT WOODS HOLE AND CORONA DEL MAR

¹ From the William G. Kerckhoff Laboratories of the Biological Sciences, California Institute of Technology, and the Marine Biological Laboratory, Woods Hole, Mass.

A repetition of some of the earlier work on Cionas at Woods Hole (in 1904, 1905, 1910), with improvements in the technique that the later work at Corona del Mar had shown to be important, has made it clear, so far as cross-fertility and self-sterility are involved, that the Woods Hole type gives the same results as the California type. The very eccentric results concerning normal and abnormal development shown in the earlier experiments appeared again, and were found not to be due entirely to differences in technique, but to differences in the eggs of the Cionas themselves, connected, in part at least, with immaturity of the animals, even when their eggs and sperm appeared to be normal, and when the eggs cleaved normally, at least into two cells.

Some of the later work, carried out at Corona del Mar in 1940 when greater care was taken in removing the egg-water (by washing the eggs) and also in removing the excess of the sperm suspension, is reported. In most cases the eggs were fertilized en masse, and, after washing again and removing the excess of water, a drop or two only of the eggs was added to the sea water (10 to 20 cc.) in Stender dishes. The reciprocal cross was made in the same way. It was found that there is a marked tendency for all the dishes of the same cross to give closely the same percentages of normal tadpoles, but there were occasional dishes that gave more extreme variations (usually more abnormals). These are due to environmental factors. The cross, when compared with its reciprocal, frequently gives different percentages of normal development when the external conditions (water and dishes) are as nearly the same as possible. Evidently, then, the eggs of different individuals in reciprocal crosses may give different percentages, although from a genetic (chromosomal) point of view the two are, on the average, identical after fertilization. It seems to follow, then, that this difference must lie in the cytoplasm of the eggs which in each case has been formed under the diploid condition of the eggs. There is also other evidence supporting such an interpretation. It should be pointed out, however, that, as a rule, the percentages of the two reciprocals are the same or nearly so. When they differ more markedly there are no definite ratios between them, so far as the observations go.

A number of experiments were also made to test whether differences in the dishes, used in the experiments after washing in tap water and drying, are responsible for the variability sometimes found in the same series. There is no evidence that this is the case if the dishes have been carefully washed and drained. On the other hand, if they have been cleaned by the ordinary chemical treatments there is evidence of effects on the development unless great care is taken to remove every trace of the cleaning fluid.