CHROMOSOMES OF ARTIFICIALLY ACTIVATED EGGS OF URECHIS ALBERT TYLER (Prom the William G. Kcrckhoff Laboratories of the Biological Sciences, Cali-fornia Institute of Technology, Pasadena, California and the William G. Kerckhoff Marine Laboratory, Corona del Mar, California) The eggs of Urcchis that cleave and develop as a result of activation by dilute sea water have been previously shown (Tyler, 193 la) to be those which extrude no polar bodies. It would appear then that the embryos produced by such eggs might be tetraploid, diploid, or haploid, depending upon the behaviour of the chromosomes during the first two nuclear divisions. A cytological investigation of such eggs shows that the embryos are diploid in chromosome number, and that only one maturation division occurs. The preparations were made by a method used by Karl Belaf and similar to that described by him (1928). It consists of joining and later separating two cover-slips, one containing a drop of eggs and the other a drop of fixing fluid. The eggs are flattened to any desired extent and stick to the cover-slips, which can be handled in the same manner as slides containing sectioned material. Two types of eggs are produced as a result of activation with dilute sea water (Tyler, 1931a). In one type the initial behaviour is identical with that of the normally fertilized eggs, two polar bodies are produced but none of the eggs divide. .In the other type the initial behaviour is quite different from that of normally fertilized eggs; no polar bodies are produced but practically all the eggs of this type divide and develop. In making the cytological preparations of the eggs of the first type, use was made of the fact, previously reported (Tyler, 1931&), that an inverse relation exists between the total percentage of activation and the percentage of cleavage. Thus treatments giving 100 per cent activation produce only eggs of the first type which do not divide. For prepara-tions of the type which does not extrude polar bodies, the eggs had to be isolated from dishes containing also unactivated eggs and activated eggs of the first type. These eggs can be readily distinguished at an early stage and can be removed for cytological preparations before the time at which the first polar body appears in the eggs of the first type. The counts of chromosome number were generally made from polar views of anaphase groups inasmuch as precociously divided chromosomes in metaphase might cause difficulty. 212
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