THE CONDITIONS THAT LEAD TO NORMAL OR ABNORMAL DEVELOPMENT OF CIONA

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Abstract
Eggs of Ciona that develop in covered Syracuse dishes produce as a rule normal tadpoles in the course of 24 hours, but occasionally only abnormal embryos develop, or both normal and abnormal in the same dish. Reciprocal crosses sometimes give very different proportions of normals and abnormals, but the relations are often very inconsistent. The results cannot be ascribed to the eggs alone or to the sperm or to their combinations. Even if an attempt is made to refer the outcome of the reciprocal crosses to the cytoplasm of the egg, that has developed under the influence of the diploid nucleus of the egg, still the differences cannot be satisfactorily accounted for. Polyspermy can account for only a very small percentage of abnormal development, probably not more than half of one per cent. Delay in fertilizing the eggs does not cause abnormal development. Delay in using the sperm suspension does not cause abnormals. When the eggs of one individual are fertilized by the same amount of sperm suspension of six other individuals, the outcome may be normals in each dish or normals and abnormals, or occasionally, all are abnormals. When the sperm of one individual is used to fertilize the eggs of six others, the same kind of results may happen. In order to make the samples of eggs as nearly alike as possible, they were first fertilized in a large volume of water, and, when in the two-cell stage, a few were transferred to several small Syracuse dishes. The same kind of irregularities appeared, which cannot be due to chance selection of different eggs, for the original eggs left in the large amount of water generally gave more than 95 per cent normals. Therefore, several kinds of experiments were made to find out whether differences in the small dishes will account for the occasional, but persistent, appearance of abnormals. (1) Dirt or impurities in the sea water from the tap was excluded. (2) Evaporation of the sea water in the covered Syracuse dishes was not enough to cause abnormal development. (3) Ordinary washing of the dishes, even rinsing them in distilled water did not remove the cause of abnormal development in some of the dishes. (4) But sterilizing the dishes in an autoclave removed the cause of most of the irregularities. It follows that the cause of the exceptional cases of abnormals is due to some contamination that remains in the dishes after washing them in fresh tap water. Since the contamination does not affect the early development of eggs (cleavage), but only later stages, it must be putrefactive in origin; and since it is removed by autoclaving the dishes, it is probably bacterial or some sort of organic contamination. The eggs come to rest on the bottom of the dishes in a few minutes where they remain until the tadpole emerges, hence local differences may affect the development and account for those cases where both normal and abnormal development takes place in the same dish. The discovery of the cause of the occasional abnormal development removes the possibility that it is due to genetic factors, hence is not concerned with the self-sterility of Ciona.

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