Diagram of the formation of spermatozoa in Ascaris megalocephala, var. bivalens. (Modified from O. Hertwig.) A. Primitive sperm-cells. B. Sperm-mother-cells. C. First nuclear division. D. The two daughter-cells, or spermatocytes. E. Second reducing' division. F. The four grand-daughter cells, the sperm-cells, or spermatides, each becoming a spermatozoon.

To face p. xxviii.

Diagram of the formation of ova in Ascaris megalocephala, var. bivalens. A. The primitive germ-cell. B. Fully developed germmother-cell, with chromosomes doubled. C. The first nuclear division. D. One daughter-cell (oocyte) extruded as the first polar body. E. The second or reducing division; the retained, and the extruded oocyte, dividing, and forming four grand-daughter cells (oozoa). F. The ripe egg-cell, the functional oozoon; the other oozoa, 2, 3, and 4, being the polar bodies.

second polar body, and one oozoon is left containing the remaining two chromosomes.

Assuming that the egg of one of the parthenogenetic species of Cynipidae had the same number of chromosomes as the species of Ascaris just described, four chromosomes would be extruded as the first polar body and four would remain in the nucleus. In that of a sexual species, on the contrary, six chromosomes would be found in the polar bodies and only two in the nucleus; but when fertilization took place, two chromosomes would enter the ovum with one spermatozoon, and six chromosomes would be added to the polar bodies when three other spermatozoa united with them. Thus the polar bodies of one egg of the parthenogenetic generation would contain four chromosomes available for the supply of germ-plasm to the next generation; while the polar bodies of a sexual egg would afford twelve chromosomes for the germ-plasm of the generation that follows. It is clear that if the polar bodies contain the germ-plasm of the next generation they must be fertilized by three spermatozoa, since in the sexual generation male and female characters are equally transmitted. If this be the case, we ought to find the eggs of the parthenogenetic generation much more numerous than those of the sexual, because the parthenogenetic germ-tracks have received the produce of twelve, while the germ-tracks of the sexual generation have only received that of four chromosomes; and it is to be presumed that the oval produced from them would bear something like this proportion to each other, if this theory held good. What are the facts? The summer sexual generations, whose germ-plasm has been received from the parthenogenetic polar body, produce from 200 to 400 eggs; while the winter agamous generation, which receives its germ

plasm from the fertilized polar bodies of the sexual generation, produces from 1,000 to 1,200 eggs; or in an approximate proportion to the number of chromosomes postulated as present.

The impression which long prevailed that one spermatozoon was equal potentially to one ovum, led observers to regard the presence of more than one spermatozoon as an act of 'polyspermy,' as abhorrent to nature, and a cause of monstrosity1. But Kupffer, Benecke and others record the fact that spermatozoa do enter the 'peculiar protoplasmic protuberances,' many appearing to form pronuclei after gaining access to the ovum, so that in numerous species polyspermy appears to be the rule.

A great deal has recently been done in working out the development of the reproductive rudiment in insects, and it seems that the cells which become the ova, and which I believe to be chiefly those primarily set aside in the polar bodies with or without corresponding spermatozoa, can be traced to the rudiment of the germ and sperm-tracks of the embryo. These germinal trackcells, formed by the first segmentation, surround the primitive germ-cells to form with them the rudimentary reproductive organs. In the germogen one germ-cell and a nutritive circle of germinal epithelium cells form a cluster, and these grow forward together, the germinal epithelium growing in between each batch. In the upper part of the developing egg-tube, each batch is very small, but as it advances it increases steadily in size. In the egg-tube of the Cynipidae the egg-stalk curves round the next egg, and the clubbed end of the stalk gives the appearance of a large and small egg alternating.

In some asexual species of Hemiptera the oocyte, Selenka, E., Befruchtung des Eies, 1871.

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which would become in an agamous species the first polar body, is not actually extruded but may be seen as a smaller cell lying close to the nucleus; and in some sexual eggs, both polar bodies are similarly retained.

It will be apparent how simply this view of the functions of the polar bodies in Cynipidae accords with the facts of alternating generations.

The fly which emerges from the gall of Spathegaster baccarum in June is sexual, and lays an egg which extrudes two polar bodies. The germ-plasm in these polar bodies after being united with that of three spermatozoa, is received by the embryo germ-tracks of this egg, and when from that egg the agamous Neuroterus lenticularis emerges in April, it is this germ-plasm which forms the nuclei of the ova contained in its tubes, consequently these ova can only reproduce the sexual flies of Spathegaster baccarum.

Again, the agamous Neuroterus lenticularis lays an egg which extrudes one polar body. The germ-plasm in this polar body is received by the germ-tracks of the embryo, and, when from that egg the sexual Spathegaster baccarum fly emerges, it is this germ-plasm which forms the nuclear matter of the ova and spermatozoa contained in its tubes, and consequently these ova and spermatozoa can only reproduce the agamous fly of Neuroterus lenticularis.

The two streams of germ-plasm are thus going on independently, and are each capable of acquiring and accumulating beneficial variations, so that the general dictum of Weismann is correct: 'the basis of the alternation of generations as regards the idioplasm, must, in all cases, consist of a germ-plasm composed of ids of at least two different kinds, which ultimately take over the control of the organism to which they give rise.' In