1) When the development of one or more blastomeres of the eggs ofAscaris megalocephala in 2- or 4-cell stages, is prevented by exposure to the rays of ultra-violet light for 6 to 8 hours, the remaining blastomeres produce the same cells and cell-groups as they would have produced had the whole egg developed normally. 2) When one blastomere of a 4-cell stage is prevented from developing, the other three may produce an approximately normal 3/4 embryo, defective as to the cells and cell-groups which should have come from the undeveloped blastomere. 3) One blastomere of a 2-cell stage does not give anything approaching in form a normal 1/2 embryo. The animal blastomere produces a blastula composed of similar cells which in the latest stages closely resemble normal ectoderm cells; and the vegetative blastomere gives a solid mass of cells, consisting of primitive germ cells, endoderm, stomodaeum, mesoderm and tail cells in approximately normal relative positions. 4) Exposure of the whole egg to ultra-violet light for 6 to 8 hours does not usually kill the egg at once, hut prevents further development. A mitosis may he completed after such an exposure, and the cells of the 4-cellT stage usually change their relative position in the shell. 5) Exposure for a period too short to prevent further cleavage (1/2 to 3 hours) causes various irregularities in development, the most frequent of which are, 1) irregular fragmentation of the chromosomes, 2) delay in cleavage, most marked in the vegetative half of egg and embryo, 3) production of long worm-like embryos with too few cells, and consisting of an elongated animal blastula with a mass of large vegetative cells at one end, 4) too many cells in which the chromosomes have not undergone diminution in some embryos, and other embryos with no germ cells whatever. 6) So far as can be seen, chromosomes in mitosis remain unchanged during exposure to ultra-violet light. They appear perfectly normal in preparations fixed immediately after exposure; or, in case of long exposure, when fixed before disintegration of the cytoplasm sets in.