A, egg surrounded by spermatazoa, one penetrating the membranes, the cytoplasm sending at “p” a hill-like processus to meet the spermatozoön; B, the tail of the spermatozoön is vanished, the sperm nucleus preceded by its centrosome is moving towards the egg nucleus which shows a chromatin reticulum; C, egg nucleus and sperm nucleus are near each other, between them the aster fibrils; D, the centrosome has divided, the chromatin has taken the form of chromosomes; E, first cleavage of the chromosomes, the splitted chromosomes are lying on the equatorial line; F, the cleavage is complete, the two-celled stage; s, spermatazoa; on, nucleus of the ovum; p, hill-like processus; sn, spermatozoön-nucleus; sc, spermatozoön centrosome; cn, chromatin-net; af, aster fibrils; cho, chromosome of the ovum; chs, chromosome of the spermatozoön; dc, divided centrosome; spc, splitted chromosomes, the lighter that of the spermatozoön and the darker those of the ovum; nn, new nuclei, each containing four chromosomes, two from the ovum and two from the spermatozoön.] This law, that the determiner in the protoplasm of the parent cell, or rather in the nucleus of the cell, always fixes the character of the progeny, holds good only of the unit character. Many individual characters are fluctuations and play no part in Mendelian heredity. Bodily modifications, resulting from environing conditions, are not Mendelized. Most of the human traits Mendelize, such as stature, span, size of head, shades of color of hair and eyes, hair curliness, pulse rate, digestion and the psychic traits, such as determination, cheerfulness, alertness, resistance to fatigue. Some anomalies also depend upon the determiners and Mendelize, such as colorblindness, ness, night blindness, albinoism, brachydactylism (only two finger joints instead of three), syndactylism, polydactylism, keratosis, hemophilia, cataract, deaf-mutism, imbecility, Hutchinson’s chorea, epilepsy, and some forms of insanity. The Mendelian law of segregation has somewhat lifted the veil in which maturition was wrapped. Every germ cell, being a product of two parent cells, is in one way or other a hybrid. By casting off half of the chromosomes during its maturing, it becomes pure. The gamete is thus always pure and ready for impregnation. _Impregnation._—The process of impregnation is about the same in most many-celled animals. As soon as the head of a single spermatozoön enters the egg-cytoplasm, a new membrane is formed around the ovum which effectually prevents the entrance of any other spermatozoa. The head and the middle piece penetrate now into the egg, the tail usually remaining imbedded in the membrane where it soon degenerates. A few moments after the spermatozoön has entered the egg, a system of radiation appears around the middle piece which develops into an aster, surrounding the centrosome of the sperm cell (Cut 29, Fig. B.).