The germinal cell divides into a number of cells called embryonic, which become differentiated into layers or groups which later on form the different organs of the body. The embryonic period is the name given to the period between the exit of the germinal cell from the maternal body and the final complete development which it acquires in becoming the adult individual. During this period the organism undergoes the most singular metamorphoses. In certain cases it forms a free embryo which appears to be complete, having a special form and mode of life, but which finally becomes transformed into an entirely different sexual individual. Thus from the egg of a butterfly there first emerges a caterpillar, which lives and grows for some time, then changes to a chrysalis and finally to a butterfly. The caterpillar and the chrysalis belong to the embryonic period. During this period every animal reproduces in an abbreviated manner certain forms which resemble more or less those through which its ancestors have passed. The caterpillar, for example, resembles the worm which is the ancestor of the insects. _Haeckel_ calls this the _fundamental biogenetic law_. We are not concerned here with embryology, and will content ourselves with some of the main points. =Germinal Cells. Hermaphrodites.= We now come to _conjugation_. In order to avoid complications we will leave aside plants and speak only of animals. Among multicellular animals, sometimes in the same individual, sometimes in different individuals, occur two kinds of sexual glands, each containing one kind of cells--the male cells and the female cells. When both kinds of sexual glands occur in the same individual, the animal is said to be _hermaphrodite_. When they develop in two different individuals the animals are of distinct sexes. Snails, for example, are hermaphrodite. There also exist lower multicellular animals which reproduce by budding, but among which conjugation takes place from time to time. We shall not consider these animals any further, as they are too remote to interest us here. =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.