In many animals it is provided with a sheath or membrane of its own; in others it remains naked, and in that case frequently displays the remarkable movements of protoplasm. The germ-cells of different classes of animals. vary considerably in their microscopic dimensions; nevertheless, in the whole animal kingdom, from the sponges and polypes up to the mammals inclusive of man, they are essentially similar. Nor do non-essential differences appear until the primitive germ-cell is more abundantly provided with yelk and albumen, and has surrounded itself with a specially thick and perforated shell, as in insects and fishes, or with a peculiarly formed sheath, in the shape of a double concave lens, as, for instance, in some Turbellaria. As a rule, the ova are formed in special organs, the ovaries. The other sexual element, the sperm, contains, as its peculiar active constituents, the spermatozoa (fig. 4 s), which consist of a pointed, elliptic, or occasionally of a hook-shaped,

FIG. 4.

S

head, and a thread-like body. As long as the sperm is capable of fecundation, the filamentous appendage performs serpentine movements, and the development of the spermatozoa from cells, as well as the comparison of their movements with the vibrating movements of ciliated and flagellate cells, enable us to recognize them also as modified cell structures.

The vehement dispute of last century between Evolutionists and Epigenists has now a merely historical interest. The former maintained that either in the ovum or in the sperm-corpuscle the whole future organism

was prefigured in all its parts, and that it hence required. only the development of the infinitely minute organs. already existing. The others, who carried off the victory, saw in the ovum the yet undifferentiated material which subsequent to fecundation had still to be transformed into the various morphological elements and organs. But it is scarcely twenty years ago since the process of fecundation was discovered, and since it was proved that at least one sperm corpuscle, and, as a rule, several or many, must penetrate into the interior of the ovum and unite materially with its substance in order to produce an effectual fecundation.

The course of our demonstration obliges us to place sexual in sharp contrast with asexual genesis. But here, again, recent times have produced a series of equalizing and conciliatory observations which must. not be neglected by us, bent as we are on tracing the antecedents of the doctrine of evolution, and demonstrating the transition taking place throughout organic Nature. In the cases of alternate generation selected above, the generations which do not produce ova and spermatozoa, reproduce themselves by external gemmation. Now, there is manifestly no great physiological difference if the deposition of the material from which the progeny is formed takes place, not externally, but in and by special internal organs. One of the most familiar examples occurs in the evolutionary cycle or alternate generation of the genus Distoma of the Entozoa. In the ventral cavity of one larval generation arise cell-spheres, or germs, which develope into the second generation-the Cercaria.

Great excitement was likewise aroused by the dis

DEVELOPMENT OF UNFERTILIZED OVA.

47

covery of the germ-formation of the larvæ of a dipterous insect (Cecidomyia, Miastor). In the ventral cavity of the maggots of these flies arises a second generation of maggots, of which the origin was primarily attributed to a simple germ-formation, until it was shown that these germs proceed from the situation of the sexual glands (which in many insects are developed at a very early stage), and must therefore be regarded as unfertilized ova. The second generation of maggots lives at the expense of its parent, consumes its fatty substance, and afterwards destroys the other organs; while of the pelican-like parent nothing finally remains but the skin, as a protecting cover to the offspring, which very soon emerges.

Without mentioning other cases in which it may be questionable whether germs or unfertilized ova attain development, we will point out a few of those in which development, without fecundation, is established with complete certainty. The queen bee, partly from the natural course of its life, partly from various accidents in which fecundation could not take place, lays regularly a number of unfertilized eggs, from which issue drones, or male individuals; or if exceptionally eggs are laid by workers, which are imperfectly developed female bees not susceptible of fecundation, these eggs likewise produce drones only. Von Siebold's highly interesting experiments on the reproduction of a wasp (Polistes Gallica), have shown that the hybernating fertilized females, who found a new colony in the spring, deposit eggs whence issue female individuals, and occasionally males. This virgin generation then produces eggs from which males are developed. With various butterflies,

on the contrary, the unfertilized eggs produce females only; and it is the same with several of the lower crustaceans.

We will now revert to the consideration of the evolutionary processes displayed in sexual reproduction after fecundation has taken place. Development invariably commences with a process of cell-formation, the bifurcation or formation of the germinal membrane, after the completion of which, instead of the one primitive cell, a large number of cells are usually in existence, as the material for the distribution and construction of the embryo. Ova developing parthenogenetically, without fecundation, likewise commence their development by this multiplication of cells; and even the ova of animals, in which development never takes place without previous fecundation, exhibit an incomplete bifurcation, if not fertilized at a certain stage of maturity. This process, it is true, has been as yet demonstrated only in the ova of the frog and the domestic fowl; but these cases are sufficient to divest the bifurcation of the character of an independent phenomenon, exclusively restricted to sexual reproduction.

8

Even before the appearance of C. E. von Baer's really classical and fundamental work on the "Evolutionary History of Animals" (Entwickelungsgeschichte der Thiere), the view, founded on incomplete observations, had become established, that in the various stages of their development the higher animals passed through the forms of the lower ones. In this, natural philosophy did not confine itself to the limits of the types; and hence did not pause at the hypothesis that the mammalian embryo was successively a fish, an amphibian,

TYPES OF DEVELOPMENT.

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and in a certain sense, by a particular gradual evolution of the organs, a bird also, but made the embryo likewise repeat and surpass the lower types. To this false tendency, acting on vague analogies, a stop was put by the great naturalist just named. He showed that a number of coincidences might, indeed, be demonstrated between the embryo of the higher and the permanent form of the lower animals, but that this resemblance rested essentially on the fact that in the embryo of the higher animal the differentiation of the general fundamental mass had not yet set in, and that in the progress of development it passes through stages which are permanent in the series of inferior animals.

On the other hand, he positively repudiated the assertion that the embryos of the higher types actually pass through forms permanent in the lower ones. He says that the type of each animal seems from the first to fix itself in the embryo, and to regulate its whole development. As regards the vertebrate animals in particular, the further we go back in the history of their development, the more do we find the embryos alike, both on the whole and in the individual parts. "Only gradually do the characters appear which mark the greater, and later those which mark the smaller divisions of the Vertebrata. Thus from the general type. the special one is evolved."

Von Baer thus held that the analogy consisted only in the embryonic states of the various animal forms; but he was obliged to go beyond the circle of the types, and he thought it probable that among all embryos of vertebrate, as well as invertebrate animals, developed from a true ovum, there is a conformity in the condition of the