surround the ova in varying numbers. Each attacks the ovum, gyrating on the posterior flagellum with a “corkscrew” action, from time to time bunting against its surface. When many sperms attack one ovum the lashing of the flagella causes it to rotate spasmodically, sometimes quite fast, but very many sperms do not rotate the ovum, only moving it slightly to and fro. The rotation is merely an incidental, mechanical effect of the great activity of the flagella of the attacking sperms. Sperms also accumulate round immature ova and foreign bodies in the culture, so it cannot only be chemotaxis which accounts for their attraction to the mature unfertilized ova.

After a variable length of time, up to half an hour, one sperm penetrates into the ovum and the remaining sperms undergo a great change. Sometimes, when only a few sperms are present, the remainder swim away from the ovum, which no longer attracts and sperms. Usually, however, when many surround the ovum and one fertilizes it, the rest of the sperms become sluggish, as if affected by a toxic influence, and they soon die. Vast numbers of dead and dying sperms may be seen at the same time in a culture, but the mortality is not altogether simultaneous, as adjacent ova may be surrounded, one by active sperms, and the other by dead and dying sperms. After a short time all the sperms die in parts of the culture dish where ova have been fertilized. It is probable that the newly fertilized ova exert some influence over the sperms in their proximity, and this conclusion is strengthened by the fact that sperms released at the same time in water containing no ova remain alive long after those added to water containing ova.

If, five minutes after mixing ova and sperms, when only a few ova will have been fertilized, the sea-water medium is greatly diluted with fresh water, most of the ova swell and disintegrate diffusely, indicating that they were naked protoplasts. A few burst at one point indicating the presence of a bounding membrane. Ten minutes after mixing, similar treatment causes considerably more ova to burst at one point. It is evident that on fertilization, a membrane is immediately secreted by the zygote. At first it is very fine and only distinguishable by shrinking or swelling the enclosed protoplast, but later it thickens enough to be easily visible. (Text Fig. 49.)

After fertilization, other changes occur. The “granular,” finely alveolar, dense layer of cytoplasm round the central nucleus becomes more apparent and the phaeoplasts become orientated radially from the boundary of this layer. They form a dense aggregation occupying about one-half the diameter of the zygote, and extend radially between the large vacuoles of the “foamy” cytoplasm. In material fixed quarter of an hour after ova and sperms were mixed, sections of zygotes are easily distinguishable from those of unfertilized ova, even if the actual sperm is not visible, because of this characteristic approach to radial arrangement and aggregation of the phaeoplasts.

Stages in the passage of the male nucleus to the female nucleus within the ovum are shown in Text Figs. 41–43 from material fixed half an hour after ova and sperms were mixed. The central