Fig. 31. Small portions of the ovum of Asterias glacialis. The Spermatozoa are shewn enveloped in the mucilaginous coat. In A. a prominence is rising from the surface of the egg towards the nearest spermatozoon; and in B. the spermatozoon and prominence have met. (Copied from Fol.)
Fig. 32. Portion of the ovum of Asterias glacialis after the entrance of a spermatozoon into the ovum. It shews the prominence of the ovum through which the spermatozoon has entered. A vitelline membrane with a crater-like opening has become distinctly formed. (Copied from Fol.)
It will be convenient again to take Asterias glacialis as the type. The part of the germinal vesicle which remains in the egg, after the formation of the second polar cell, becomes converted into a number of small vesicles ([fig. 29]), which aggregate themselves into a single clear nucleus, which gradually travels toward the centre of the egg and around which, as a centre, the protoplasm becomes radiately striated ([fig. 30]). This nucleus is known as the female pronucleus. By the action of reagents a nucleolus may be shewn in it. In Asterias glacialis the most favourable period for fecundation is about an hour after the formation of the female pronucleus. If at this time the spermatozoa are allowed to come in contact with the egg, their heads soon become enveloped in the investing mucilaginous coat. A prominence, pointing towards the nearest spermatozoon, now arises from the superficial layer of protoplasm of the egg, and grows till it comes in contact with the spermatozoon ([fig. 31], A and B). Under normal circumstances the spermatozoon which meets the prominence is the only one concerned in the fertilization, and it makes its way into the egg by passing through the prominence. The tail of the spermatozoon, no longer motile, remains visible for some time after the head has bored its way in, but its place is soon taken by a pale conical body, which is, however, probably in part a product of the metamorphosis of the tail itself ([fig. 32]). It eventually becomes absorbed into the body of the ovum.
At the moment of contact between the spermatozoon and the egg the outermost layer of the protoplasm of the latter raises itself as a distinct membrane, which separates from the egg and prevents the entrance of other spermatozoa. At the point where the spermatozoon entered a crater-like opening is left in the membrane, through which the metamorphosed tail of the spermatozoon may at first be seen projecting ([fig. 32]).
Fig. 33. Ovum of Asterias glacialis, with male and female pronucleus and a radial striation of the protoplasm around the former. (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, 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 amœboid contractions, and eventually fuses with it ([figs. 34]‑36).