Fig. 13.
Figs. 12 and 13.—Small portion of the ovum of Asterias glacialis. The spermatozoa are shewn enveloped in the mucilaginous coat. In Fig. 12 a prominence is rising from the surface of the egg towards the nearest spermatozoon; and in Fig. 13 the spermatozoon and prominence have met. From living ovum (copied from Fol).
At the moment of contact between the spermatozoon and the egg the outermost layer of the protoplasm of the latter raises itself as distinct membrane, which separates from the egg and prevents the entrance of any more spermatozoa. At the point where the spermatozoon entered a crater-like opening is left in the membrane (Fig. 14).
Fig. 14.—Portion of the ovum of Asterias glacialis after the entrance of a spermatozoon into the ovum. It shows the prominence of the ovum through which the spermatozoon has entered. A vitelline membrane with a crater-like opening has become distinctly formed. From living ovum (copied from Fol).
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 by absorbing, it is said, material from the ovum, though this may be doubted, 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. 15). At whatever point of the egg the spermatozoon may have entered, it gradually travels towards the female pronucleus. This latter, around which the protoplasm no longer has a radial arrangement, remains motionless till it comes in contact with the rays of the male pronucleus, after which its condition of repose is exchanged for one of activity, and it rapidly approaches the male pronucleus, and eventually fuses with it (Fig. 16).
Fig. 15.—Ovum of Asterias glacialis, with male and female pronucleus and a radial striation of the protoplasm around the former. From living ovum (copied from Fol).
Fig. 16.—Three successive stages in the coalescence of the male and female pronucleus in Asterias glacialis. From the living ovum (copied from Fol).