Page:Popular Science Monthly Volume 62.djvu/309

Rh Sphaerella a large number of gametes (32-64) are formed in a mother cell. These sexual cells are quite similar in size and form, and when they conjugate no one could assert a difference in sex. Most species of Chlamydomonas resemble Sphaerella in having gametes essentially

similar (see Fig. 2, a), but there are forms in this genus presenting a marked advance. In Chlamydomonas Braunii the gametes are of two sizes and the smaller always unite with the larger. However, both are ciliate and consequently motile, so that morphologically they are similar, although there can be no doubt of the sexual differentiation. It is not customary to call the female cell an egg until it has lost its free swimming possibilities and as a passive cell awaits the specialized motile sperm.

Pandorina, like Sphaerella, produces a large number of gametes, 16-32 in the mother cell, but here there is a considerable range of variation in the size of the sexual elements, although the form is always the same. Sometimes these gametes will pair, a small one with a larger (see Fig. 2, b), as in Chlamydomonas Braunii, thus showing the tendency towards sexual differentiation. However, there is no rule in this habit of Pandorina, for frequently gametes of equal size conjugate, and one can not assert that the larger cells are always destined to be female sought by smaller male elements. What determines the variation in size of the gametes in Chlamydomonas Braunii and Pandorina, and the consequent differentiation in sex? It. depends entirely on the number of gametes formed in the mother cells and on the size of the latter. The larger female gametes are formed less numerously and generally in larger mother cells and