The head of the spermatozoon when in the egg forms a nucleus, for which the name male pronucleus may be conveniently adopted. It grows in size, probably by assimilating material from the ovum, and around it is formed a clear space free from yolk-spherules. Shortly after its formation the protoplasm in its neighbourhood assumes a radiate arrangement (fig. 33). At whatever point of the egg the spermatozoon may have entered, it gradually travels towards the female pronucleus. The latter, around which the protoplasm no longer has a radiate arrangement, remains motionless till the rays of the male pronucleus come in contact with it, after which its condition of repose is exchanged for one of activity, and it rapidly approaches the male pronucleus, apparently by means of its inherent amoeboid contractions, and eventually fuses with it (figs. 34-36). As the male pronucleus approaches the female the latter, according to Selenka, sends out protoplasmic processes which embrace the former. The actual fusion does not take place till after the pronuclei have been in contact for some time. While the two pronuclei are approaching one another the protoplasm of the egg exhibits amoeboid movements. [FIGS. 34, 35, AND 36. THREE SUCCESSIVE STAGES IN THE COALESCENCE OF THE MALE AND FEMALE PRONUCLEI IN ASTERIAS GLACIALIS. FROM THE LIVING OVUM. (Copied from Fol.)] The product of the fusion of the two pronuclei forms the first segmentation nucleus (fig. 37), which soon, however, divides into the two nuclei of the two first segmentation spheres. [FIG. 37. OVUM OF ASTERIAS GLACIALIS, AFTER THE COALESCENCE OF THE MALE AND FEMALE PRONUCLEI. (Copied from Fol.)] The phenomenon which has just been described consists essentially in the fusion of the male cell and the female cell. In this act the protoplasm of the two cells as well as their nuclei coalesce, since the whole spermatozoon which has been absorbed into the ovum is a cell of which the head is the nucleus. It is clear that the ovum after fertilization is an entirely different body to the ovum prior to that act, and unless the use of the same term for the two conditions of the ovum had become very familiar, a special term, such as oosperm, for the ovum after its fusion with the spermatozoon, would be very convenient.