sizes (Fig. 7, D). These coalesce, round up into a spherical cell (Fig. 7, E, F), which soon develops an enveloping cellulose wall, and passes as a "zygote " into a resting stage (Fig. 7, G). In this condition the organism.

may remain dormant for a long time, thus tiding over a period of drouth, the winter months, etc. After this resting period, if brought under suitable conditions of moisture, the outer wall of the zygote ruptures, the contents escape in the form of a large swarm spore, which swims about for a time and then divides into the sixteen cells of a new colony (Fig. 7, H, I, J).

Reproduction in
Eudorina.

In Eudorina elegans, a form closely related to Pandorina, there is a striking difference in the size of the conjugating zoospores. In this form sixteen or thirty-two cells are imbedded together in a common spherical gelatinous mass. The asexual mode of reproduction is the same as in Pandorina, just described, each cell of a colony being transformed by successive divisions into a new colony of sixteen or thirty-two cells which becomes free from the parent colony. The sexual mode presents a difference in that the colonies differentiate into two sorts termed male and female. In the female colonies the cells become transformed into spherical egg cells or oospheres without further division. In the male colonies, however, each cell divides into sixteen or thirtytwo antherozooids, minute, elongated cells, each provided with two long cilia projecting from its anterior end (Fig. 8, A, B, C). These remain slightly united together in bundles and, escaping from the parent colony, swarm for a time in the water together. Coming in contact with a colony of oospheres, they break apart, penetrate into the gelatinous envelope, and find their way to the egg cells (Fig. 8, D). A single antherozooid fuses with each egg cell, and the conjugated pair form a resting zygote around which a cellulose wall forms, and from which, after a certain period of time, a new colony of sixteen or thirty-two cells develops.

A third stage in the differentiation of the conjugating

reproductive cells is found in Volvox globator. This form consists of a hollow spherical colony of as many as twenty-two thousand cells placed in a single layer in a hyaline jelly-like substance, and connected with each other by cytoplasmic processes. Each one of the cells is a somewhat ovoid

Volvox.

ע

B

FIG. 8.-Eudorina elegans, a female colony around which antherozooids are swarming: A, cluster of antherozooids still united; B, cluster of antherozooids just separating; C, swarming antherozooids, some of which have already penetrated into the female colony D. (After Goebel.)

mass of green-coloured protoplasm, and bears two long cilia upon its outer, pointed, hyaline end which project out into the water and, lashing to and fro, give to the whole colony a rotary motion. At the time of reproduction, certain cells of the colony undergo profound modifications. Some of them increase in size enormously, having reserve food material stored up in them, and become the egg cells or oospheres. Other cells divide

into bundles of minute antherozooids (sixty-four to a hundred and twenty-eight). The remaining cells of the colony, remain in a vegetative condition, and eventually die. In reproduction, one of the antherozooids fuses with one of the oospheres, a resting zygote is formed from which develops later a new colony. Thus in the Volvox colony we meet with a differentiation into somatic or vegetative cells and reproductive cells, a differentiation which persists through all the multicellular plants and animals.

A much larger series of forms might be cited to illustrate the phenomena of multiplication among unicellular organisms, which would show all stages of gradation in the relative size of the conjugating cells from those in which both are of equal size and are equally active, to such forms as Volvox, in which a great difference in size exists, the larger, the oosphere, being non-motile and laden with food material, the smaller, the antherozooid, having the cytoplasm reduced to a very small amount and being endowed with high mobility.

Reproduction in
Metazoa.

In multicellular organisms we meet with a continuation of the same facts. The animal egg is a single cell laden with a large amount of food yolk, and made up of nucleus and cytoplasm as the living elements. For the development of this egg, conjugation with another germ cell, derived from a different individual, is necessary. This germ cell is the spermatozooid, a minute cell consisting of nucleus and centrosome with a small amount of cytoplasm modified primarily into an organ of locomotion, the tail. A physiological division of labour is here met with which admirably meets two diametrically opposed requirements. The one of these demands that the conjugating cells be highly motile, and consequently small,

in order that they may be able to come together in the water in which they are usually set free. The second requires that there be furnished a sufficient amount of nutritive material for the nourishment of the embryo until it arrives at a stage of growth in which it can shift for itself. These two necessities have been met by a physiological division of labour between the two conjugating cells. The one, the sperm cell, has become reduced in size with a corresponding gain in motility, the other, the egg cell, has had food yolk stored up in it, and its consequent increased size prevents any more than a very slight degree of independent movement, if any. Different stages of these modifications may be met with among unicellular forms, as illustrated above in Pandorina, Eudorina, and Volvox, to which might be. added many others. In Pandorina the conjugating cells. are of nearly equal size, in Eudorina an intermediate condition is reached, while in Volvox the egg and sperm cells are sharply differentiated in size and motility. Again, in the first two and their allies all of the cells are at first vegetative and afterward reproductive, while in Volvox the definite separation into vegetative or somatic, and reproductive or germinal cells makes its appearance.

Fundamental identity of the germ cells.

We arrive then at the conclusion, from the consideration of these and many other lines of evidence, that the germ cells were primitively exactly alike, and that the differences between them have arisen in the process of differentiation along two separate lines. Furthermore, it is clear that the differences between the two sexes, which become strongly characterized in the higher vertebrates, are all of a purely secondary nature.

In their early development the germ cells are indistinguishable from each other, and both pass through