difference. In some cases, as in Daphnia, all the parthenogenetic eggs hatch into females; in other cases, as in bees, they give rise to males alone; while in still other cases, as in the gall-wasps, some of the unfertilized eggs produce males and some females.

In

In many cases the animals which are thus produced are perfectly normal, and have nothing to distinguish them from those born from impregnated eggs. They have the ordinary structure of their species, and they are perfectly capable of propagating their kind. some cases, as in the gallwasps, reproduction is preceded by the union of the sexes, and in other cases the animals born from parthenogenetic eggs are themselves parthenogenetic.

There is possibly one difference between ordinary and parthenogenetic eggs, the presence of polar globules in the one case and their absence in the other; and I shall discuss this difference soon.

Except in this particular, the history of the development of the egg into the perfect animal is the same, whether the egg is fertilized or not. Weismann, who has studied the embryology of both parthenogenetic and fertilized eggs in insects ("Beiträge zur Kenntniss der ersten Entwicklungsvorgänge im Insectenei "), shows that all the minuter details in the process of building up the embryo are the same, whether the egg is fertilized or not.

We must therefore believe that an ovum has in itself the power to give rise to a new organism, and that although it does not usually manifest this power, unless the egg is fertilized, it may exhibt it under certain circumstances, as parthenogenesis. Of the character of the circumstances which lead to parthenogenesis we know nothing, except that such circumstances have

thus acted in many groups of animals where the eggs ordinarily require to be fertilized.

Certain authors have suggested that there may be a connection between the extrusion of the "polar globules" from the ovum and the need of impregnation by a male cell.

The ripe ovarian ovum of an animal usually contains a transparent central body, the germinative vesicle, and when the egg is fully ripe the germinative vesicle approaches the surface and divides into two portions: one of these is discharged from the egg, thus forming the "polar globules." These take no part in the formation of the embryo. They become entirely separated from the egg, and soon die and disappear. The remainder of the germinative vesicle remains in the egg, as the "female pronucleus," which unites with the "male pronucleus" formed from the male cell after impregnation, and thus builds up a compound body, the first "scgmentation nucleus."

The formation of these "polar globules" has been observed in all groups of the animal kingdom, except the rotifera and arthropods, and their functional significance is therefore a subject of the greatest interest. They obviously contain something which is not needed for the formation of the embryo, and they may be discharged from the egg before it is laid, or they may remain until it is laid, as seems to be the general rule, and may be discharged just before fertilization takes place, as is the case in the star-fish, or they may be diccharged immediately after the egg is impregnated.

Within recent years an hypothesis regarding their sig-. nificance has excited considerable notice. This hypothesis, which was first advanced by the late Prof. McCrady, and which is stated at length in Balfour's Treatise on

Comparative Embryology, is that each sexual element originally contains a male portion and a female portion; the ripe male cell is the male half of the male element, and the "polar globules" contain the male substance of the ovum, which is discharged in order that it may be replaced by the male element from the body of another organism. Balfour says: "I would suggest that in the formation of the polar cells part of the constituents of the germinal vesicle, which are requisite for its functions as a complete and independent nucleus, is removed to make room for the supply of the necessary parts to it again by the spermatic nucleus. My view amounts to the following, viz., that after the formation of the polar cells the remainder of the germinal vesicle within the ovum (the female pronucleus) is incapable of further development without the addition of the nuclear part of the male element (spermatozoon), and that if polar cells were not formed parthenogenesis might normally A strong support for this hypothesis would be afforded were it to be definitely established that a polar body is not formed in the arthropoda and rotifera; since the normal occurrence of parthenogenesis is confined to these two groups in which polar bodies have not so far been satisfactorily observed. . . . To the suggestion already made with reference to the function of the polar cells, I will venture to add the further one, that the function of forming polar cells have been acquired by the ovum for the express purpose of preventing parthenogenesis. . . . There can be little doubt that the ovum is potentially capable of developing, by itself, into a fresh individual, and therefore, unless the absence of sexual differentiation was very injurious to the vigor of the progeny, parthenogenesis would most certainly be a very constant occurrence; and, on the

occur.

analogy of the arrangements in plants to prevent selffertilization, we might expect to find some contrivance both in animals and plants to prevent the ovum developing by itself without fertilization. On my hypothesis the possibility of parthenogenesis, or at any rate its frequency in arthropoda and rotifera, is possibly due to the absence of polar cells" (Comp. Emb.., vol. i. p. 63).

The simplicity of this hypothesis renders it very fascinating, but even if it were possible to accept it without qualification, it would not affect our argument, for it would still remain true that "the ovum is potentially capable of developing, by itself, into a fresh individual," and must therefore be very different in function from the male cell, which under no circumstances exhibits a similar power.

ovum.

My reasons for doubting the hypothesis are, first, that a failure to discover polar cells in the eggs of rotifera or of the arthropods may be due to the fact that they are discharged very early in the history of the ovarian We know that in some animals, as in hydra, the polar cells are discharged while the egg is still contained in the ovary, and we also know that the eggs of many arthropods undergo in the ovary very peculiar changes, which greatly obscure their fundamental similarity to ordinary uncomplicated eggs, so that it is quite possible that our failure to discover the polar cells may be due to something else than to the fact that they are never formed. The eggs of insects especially are very peculiar, and Weismann says that "nirgends im ganzen Thierreich die Ontogenese so verschoben und coenogenetisch entartet ist" as it is among the insects. This anthor has figured, in the fertilized egg of a species of Chironomus, certain bodies which are not present in the

parthenogenetic eggs of Rhodites, and he suggests that these may be the long-sought polar cells, but he does not feel certain that this is the case, and examination of his paper will show that there is so much difference between the early stages of insect eggs and the corresponding stages of simpler and more typical eggs, that the identity of these bodies must remain open to some doubt.

There is another objection to the hypothesis, which seems to me to be entitled to great weight. According to Balfour's statement we should expect that any egg which retained the polar cells might develop without impregnation. Observers have failed to discover their extrusion in the eggs of ordinary arthropods, as well as in those which are parthenogenetic, and we should therefore expect all the arthropods to be parthenogenetic, but this is not the case. In many other animals, as in the oyster, they are not discharged until the egg is fertilized, and the hypothesis would require us to believe that an unfertilized oyster egg contains a male element as well as a female element; but when perfectly ripe oyster eggs are placed, without fertilization, under conditions which are perfectly favorable to development: they show no signs of life, and soon die and decay. If a little male fluid is added, however, they quickly discharge their polar cells, and then rapidly pass through the changes which build up the embryo.

If the polar cell is really equivalent to a male cell, we certainly might expect these oyster eggs, which are perfectly ripe, and, according to the hypothesis, contain all that is necessary for development, to show some power to develop without impregnation. If the power to extrude polar cells " has been acquired by the ovum for the express purpose of preventing parthenogen