1. Ova of the mouse were fixed at times varying from 5 to 43 hours after copulation. 2. Ovulation may take place at either the stage of the first or the second maturation division. 3. The sperm enters the egg during the anaphase of the second maturation division. As indicated by one egg the sperm may occasionally enter during the first division. The second maturation division takes place between 5 to about 7 hours after pairing. The stage of the pronuclei may be reached at about 6 hours after pairing, but usually is not reached until later. The first cleavage division takes place between 21 and 28 hours. 4. During the maturation divisions the yolk-globules are most numerous in the half of the egg opposite to that at which the divisions are taking place. The mitochondria are granular and are collected into small clumps; a few, chiefly towards the periphery, are scattered singly through the cytoplasm. They are most numerous in the central region. During the first division, and in the early stages of the second division, they are numerous around the spindle. Few mitochondria are eliminated in the polar bodies. The Golgi elements are in the form of granules and of small irregularly shaped bodies; they are fairly evenly distributed through the cytoplasm. 5. During the stage of the pronuclei the distribution of the yolk-globules is similar to that during the polar divisions. In the later phases there is a concentration of the mitochondria in the neighbourhood of the pronuelei. The Golgi elements are scattered through the cytoplasm; they increase in size and undergo fragmentation. 6. The mitochondria become arranged around the spindle of the first cleavage division and are transmitted with approximate equality between the first two blastomeres. The Golgi elements remain scattered through the cytoplasm but are slightly more numerous in the neighbourhood of the zygote nucleus. The Golgi elements and the yolk-globules are fairly evenly distributed between the first two blastomeres. 7. The middle-piece of the sperm is carried into the egg. The mitochondria of the sheath are, at first, larger than those of the egg; they spread out through the cytoplasm and undergo fragmentation. It is concluded that the mitochondria of the sperm are transmitted to the first two blastomeres. There is evidence that the Golgi elements of the middle-piece fragment and that the individual elements become scattered through the ooplasm before the first cleavage division. 8. Granular mitochondria are present in the blastomeres of the two-cell stage; they are collected into small clumps and are scattered singly through the cytoplasm. The Golgi elements undergo fragmentation; they are fairly evenly distributed throughout the cytoplasm.