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.