reason for believing that sex was already developed in the fertile ovum and consequently in the resulting blastomeres from that ovum. The young of the nine-banded armadillo in a given litter are invariably of the same sex and are closely similar in all features. Newman and Patterson have shown that all the members of a litter come from the same egg. Patterson has established the fact that cleavage of the egg takes place in the usual manner, but later separate centers of development appear in the early embryonic mass and give rise to the separate young individuals. Again in certain insects where one egg indirectly gives rise to a chain of embryos, or to a number of separate larvæ, possibly as many as a thousand, all of the latter are of the same sex. Even in some plants researches have shown that sex is already determined at the beginning of development. Then, too, much evidence has come to light recently showing that sex-characters in certain cases behave as heritable characters and are independent of external conditions. Lastly there is visible and convincing evidence obtainable through microscopical observations that sex is determined by a mechanism in the germ-cells themselves. It is chiefly to these latter facts that I wish to direct attention for the present. =The Sex Chromosome.--=The evidence centers about a special chromosome or chromosome-group commonly designated as the _sex-chromosome_ or _X-element_, which has been found in various species of animals, including man. In the males of such animals this chromosome is present in addition to the regular number of pairs, thus giving rise to an _uneven_ instead of the conventional even number of chromosomes. This element remains undivided in one of the maturation divisions of the spermatocytes, in some forms in the first in others in the second, and passes entire to one pole of the spindle (Fig. 13, p. 58). This results in the production of two classes of cells, one containing the _X_-element and one not. The outcome is that two corresponding classes of spermatozoa are produced. The phenomena involved are diagrammatically represented in Fig. 13. It has been clearly demonstrated in several cases that eggs fertilized by spermatozoa which possess this _X_-element, always become females, those fertilized by spermatozoa which do not possess it always develop into males. [Illustration: FIG. 13