The adult antennal glands of Euphausiids have been described by Chun (1896), Zimmer (1913), Raab (1915), and Cannon and Manton (1927). In the course of an investigation into the development of these organs, undertaken by me at the suggestion of Professor H. Graham Cannon, it was discovered that they arise quite late in the life-history, whilst earlier larvæ were found to possess typical maxillary glands. This finding was not unexpected, for an inverse relationship between antennal and maxillary segmental excretory organs has been noted in the development of many Crustacea.

Drosophila melanogaster and D. simulans constitute one of those cases in which two almost identical phenotypes result from different genotypes. In particular, the histology of gametogenesis shows no detectable difference between the two species, nevertheless, the development of the gonads in their hybrids is so upset that invariably sterility results. In these hybrids the germ cells never go beyond the stage of oogonia or spermatogonia.

Tradescantia pollen grains are excellent material for the analysis of X-ray effects. The influence of radiation on cell development, nuclear differentiation, as well as on chromosome structure can be studied. Extensive quantitative analyses, which primarily concern the mechanism of chromosome breaks and reunions induced by radiation, have already been made (Riley, 1936; Sax, 1938, 1940, 1941; Fabergé, 1940). The present paper contains data which show the effects of radiation on cell development, chromosome behaviour, nuclear differentiation, and pollen germination. It is expected that the data obtained will throw some light on the intricate mechanism present within the cell and show that the governing centre of this mechanism is the nucleus.

In July and August 1929 the writer's father, Mr D. K. Kevan, found large numbers of a species of Lymnœa, cleaned out of a warm engine-pond in a timber yard in Leith. These snails showed much variation in the form of the shell, and this made identification difficult.

In July and August 1940 the writer revisited the locality, collected a considerable number of living snails from the pond in question, and eventually established that they belonged to the common North American species, Lymnœa (Stagnicola) catascopium, Say.

The present paper is the first step in an attempt to combine study of the genetics of coat colour in dogs with histological examination of the hairs and pigmentation, so that a picture may eventually be formed of the genetic relations of the various phenotypes, and of the processes by which the various colours are produced. Work along similar lines has been carried out by other workers on various mammals, e.g. on the mouse (Dry, 1926, 1928; Dunn, 1936; Einsele, 1937), guinea-pig (Wright and Hunt, 1918; Russel, 1939; Heidenthal, 1940), and in cattle (Bogart and Ibsen, 1938).