The works of Francis Maitland Balfour, Volume 1 (of 4)

by Francis M. Balfour

The product of this fusion forms the first segmentation nucleus (Fig. 17), which soon, however, divides into the two nuclei of the two first segmentation spheres. While the two pronuclei are approaching one another the protoplasm of the egg exhibits amœboid movements.

Of the earlier observations on this subject there need perhaps only be cited one of E. van Beneden, on the rabbit's ovum, shewing the presence of two nuclei before the commencement of segmentation. Bütschli was the earliest to state from observations on Rhabditis dolichura that the first segmentation nucleus arose from the fusion of two nuclei, and this was subsequently shewn with greater detail for Ascaris nigrovenosa, by Auerbach (1). Neither of these authors gave at first the correct interpretation of their results. At a later period Bütschli (5) arrived at the conclusion that in a large number of instances (Lymnæus, Nephelis, Cucullanus, &c.), the nucleus in question was formed by the fusion of two or more nuclei, and Strasburger at first made a similar statement for Phallusia, though he has since withdrawn it. Though Bütschli's statements depend, as it seems, upon a false interpretation of appearances, he nevertheless arrived at a correct view with reference to what occurs in impregnation. Van Beneden (3) described in the rabbit the formation of the original segmentation nucleus from two nuclei, one peripheral and the other central, and he gave it as his hypothetical view that the peripheral nucleus was derived from the spermatic element. It was reserved for Oscar Hertwig (11) to describe in Echinus lividus the entrance of a spermatozoon into the egg and the formation from it of the male pronucleus.

Fig. 17.—Ovum of Asterias glacialis, after the coalescence of the male and female pronucleus (copied from Fol).

Though there is a general agreement between the most recent observers, Hertwig, Fol, Selenka, Strasburger, &c., as to the main facts connected with the entrance of one spermatozoon into the egg, the formation of the male pronucleus, and its fusion with the female pronucleus, there still exist differences of detail in the different descriptions which partly, no doubt, depend upon the difficulties of observation, but partly also upon the observations not having all been made upon the same species. Hertwig does not enter into details with reference to the actual entrance of the spermatozoon into the egg, but in his latest paper points out that considerable differences may be observed in occurrences which succeed impregnation, according to the relative period at which this takes place. When, in Asterias, the impregnation is effected about an hour after the egg is laid and previously to the formation of the polar cells, the male pronucleus appears at first to exert but little influence on the protoplasm, but after the formation of the second polar cell, the radial striæ around it become very marked, and the pronucleus rapidly grows in size. When it finally unites with the female pronucleus it is equal in size to the latter. In the case when the impregnation is deferred for four hours the male pronucleus never becomes so large as the female pronucleus. With reference to the effect of the time at which impregnation takes place, Asterias would seem to serve as a type. Thus in Hirudinea, Mollusca, and Nematodes impregnation normally takes place before the formation of the polar bodies is completed, and the male pronucleus is accordingly as large as the female. In Echinus, on the other hand, where the polar bodies are formed in the ovary, the male pronucleus is always small.

Selenka, who has investigated the formation of the male pronucleus in Toxopneustes variegatus, differs in certain points from Fol. He finds that usually, though not always, a single spermatozoon enters the egg, and that though the entrance may be effected at any part of the surface, it generally occurs at the point marked by a small prominence where the polar cell was formed. The spermatozoon first makes its way through the mucous envelope of the egg, within which it swims about, and then bores with its head into the polar prominence. The head of the spermatozoon on entering the egg becomes enveloped by the superficial protoplasm, and travels inward with its envelope, while the tail remains outside. As Fol has described, a delicate membrane becomes formed shortly after the entrance of the spermatozoon. The head continues to make its way by means of rapid oscillations, till it has traversed about one eighth of the diameter of the egg, and then suddenly becomes still. The tail in the meantime vanishes, while the neck swells up and forms the male pronucleus. The junction of the male and female pronucleus is described by Fol and Selenka in nearly the same manner.

Giard gives an account of impregnation which is not easily brought into harmony with that of the other investigators. His observations were made on Psammechinus miliaris. At one point is situated a polar body and usually at the pole opposite to it a corresponding prominence. The spermatozoa on gaining access to the egg attach themselves to it and give it a rotatory movement, but according to Giard none of them penetrate the vitelline membrane which, though formed at an earlier period, now retires from the surface of the egg.

Giard believes that the prominence opposite the polar cells serves for the entrance of the spermatic material, which probably passes in by a process of diffusion. Thus, though he regards the male pronucleus as a product of impregnation, he does not believe it to be the head of a spermatozoon.

Both Hertwig and Fol have made observations on the result of the entrance into the egg of several spermatozoa. Fol finds that when the impregnation has been too long delayed the vitelline membrane is formed with comparative slowness and several spermatozoa are thus enabled to penetrate. Each spermatozoon forms a separate pronucleus with a surrounding sun; and several male pronuclei usually fuse with the female pronucleus. Each male pronucleus appears to exercise a repulsive influence on other male pronuclei, but to be attracted by the female pronucleus. When there are several male pronuclei the segmentation is irregular and the resulting larva a monstrosity. These statements of Fol and Hertwig are at first sight in contradiction with the more recent results of Selenka. In Toxopneustes variegatus Selenka finds that though impregnation is usually effected by a single spermatozoon yet that several may be concerned in the act. The development continues, however, to be normal if three or even four spermatozoa enter the egg almost simultaneously. Under such circumstances each spermatozoon forms a separate pronucleus and sun.

It may be noticed that, while the observations of Fol and Hertwig were admittedly made upon eggs in which the impregnation was delayed till they no longer displayed their pristine activity, Selenka's were made upon quite fresh eggs; and it seems not impossible that the pathological symptoms in the embryos reared by the two former authors may have been due to the imperfection of the egg and not to the entrance of more than one spermatozoon. This, of course, is merely a suggestion which requires to be tested by fresh observations. We have not as yet a sufficient body of observations to enable us to decide whether impregnation is usually effected by a single spermatozoon, though in spite of certain conflicting evidence the balance would seem to incline towards the side of a single spermatozoon[368].