Spermatozoa and Ova.—In all the higher animals, including the hermaphrodites, the male germinal cells, or spermatozoa are characterized by their mobility. Their protoplasm is contractile and their form varies according to the species. In man and vertebrate animals they resemble infinitely small tadpoles, and their tails are equally mobile. The female germinative cell, on the contrary, is immobile and much larger than the male cell. Conjugation consists in the movement of the male cell, by means of variable mechanism, toward the female cell, or egg, into the protoplasm of which it enters. At this moment it produces on the surface of the egg a coagulation, which prevents the entrance of a second spermatozoid.
The egg and the spermatozoid both consist of protoplasm containing a nucleus. But, while the spermatozoid has only a small nucleus and very little protoplasm, the egg has a large nucleus and a large quantity of protoplasm. In certain species the protoplasm of the egg grows in the maternal organism in a regular manner to form the vitellus (yolk of egg) which serves as nourishment for the embryo for a long period of its existence. This occurs in birds and reptiles.
Conjugation.—The phenomena of conjugation were made clear by van Beneden and Hertwig. These phenomena, as we have seen, commence among unicellular organisms. In these they do not constitute reproduction, but the vital reënforcement of certain individuals. Conjugation takes place in a different manner in different cases.
For example, a unicellular animal applies itself against one of its fellows. The nucleus of each cell divides into two. Then the protoplasm of the two cells fuses over the whole surface of contact, and half the nucleus of the first cell penetrates the second cell, while half the nucleus of the latter enters the first cell. After this exchange the cells separate from each other and each exchanged half of the nucleus fuses with the primitive half of the nucleus remaining in the cell.
From this moment each cell continues to reproduce itself by fission, as we have seen above. In another form, two cells meet and fuse completely. Their nuclei become applied against each other and each exchanges half its substance with the other as in the preceding case, so that the final result is the same. In both cases the two conjugated cells are identical, and one cannot call them male and female.
Penetration of the Spermatozoid into the Egg.—In all the higher animals in which the germinal cells are of two kinds, male and female, conjugation takes place in rather a different manner. Here, the female cell or egg only reproduces itself exceptionally by parthenogenesis. It usually contains no chromosomes and often too little chromatin, so that it perishes when conjugation does not occur.
The spermatozoid swims by means of its tail to meet the egg. As soon as it touches it it penetrates it and the coagulation which we have mentioned is produced. This coagulation forms the vitelline membrane, which prevents the entry of other spermatozoids. If, from pathological causes the entry of several spermatozoids takes place, there results, according to Fol, a double or triple monster.
In Fig. 11 on Plate II, we see the egg with its vitelline membrane and nucleus, the chromatin network of which is marked in blue: b shows the protoplasm of the egg or vitellus; a the vitelline membrane; d the spermatozoid which has just entered, and the nucleus of which, composed chiefly of chromatin, is colored red, while its tail has performed its task and is about to disappear. The letters e, f, and g, show a spermatozoid which has arrived too late.
Before the head of the spermatozoid which has entered, appears a centrosome (Fig. 12) which it brings to the egg with its small amount of protoplasm, and around this centrosome rays form, as in the case of cellular fission. At the same time a nuclear liquid arising from the protoplasm of the egg becomes concentrated around the chromatin of the spermatozoid, while the nucleus of the egg remains in place and does not change. The nucleus of the spermatozoid, on the contrary, begins to grow rapidly. It forms half the number of chromosomes corresponding to the cell of the species to which it belongs, and grows at the expense of the vitellus of the egg. During this time the centrosome divides into two halves, which progress slowly on each side toward the periphery of the egg, as in the case of fission (see Plate I), while the chromatin of the chromosomes of the spermatozoid is dissolved in the network. The nucleus thus formed by the spermatozoid enlarges more and more (Figs. 13 and 14) till it attains the size and shape of that of the egg (Fig. 15). The male and female chromatin are colored red and blue respectively.
Then only commences activity of the nucleus of the egg, at the same time as fresh activity on the part of the nucleus of the spermatozoid. Before this, however, the nucleus of the egg has thrown off a part of its chromatin called a polar body, and it now possesses only half as much chromatin as the other cells of the body of the individual. The nucleus of the egg and that of the spermatozoid then begin at the same time to concentrate their chromatin in the form of chromosomes (Fig. 16) which arrange themselves regularly in the middle line exactly as shown in Plate I, and divide longitudinally into two halves which are then attracted in opposite directions by the rays of each of the centrosomes (Fig. 17). Figure 17, of Plate II, thus corresponds exactly to Fig. 6, of Plate I.