to be only certain cells "which have the capacity of becoming ova." These are called by Mr. Balfour "germinal cells." "The mode of conversion of the germinal cells into ova is somewhat diverse." With these modes we need not further trouble ourselves. But the embryologist does not tell us wherein lies their "capacity;" yet this is what we should much like to know.

The process of fertilisation is also suggestive of difficulties which are beyond the range of existing science. Professor Häckel's hypothesis is that "growth, which is the condition necessary to production, was attained by the union of two full-grown cells into a single cell, &c. At first the two united cells may have been entirely alike. Soon, however, by natural selection, a contrast must have arisen between them. For it must have been very advantageous to the newly created individual in the struggle for existence to have inherited various qualities from the two parent cells. The complete development of this progressive contrast between the two producing cells led to sexual differentiation. One cell became a female egg-cell, the other a male seed or sperm-cell." 1 The working of natural selection is less easily followed here than might be wished. The implied principle is that, the "contrast," in cells at first "entirely alike," was the consequence of unlike exposure to incident forces; and that the new resultant, produced by fusion, profited by the education, so to speak, which the original cells had received. This principle proving more successful than parthenogenesis-self-production was seized upon by natural selection, and became a law of nature. The advantage to the "newly created individual" is intelligible enough; and natural selection would undoubtedly favour beings endowed with the capacities of two parents instead of one. But inasmuch as it is the offspring and not the maternal cells that are better provided for the struggle, one does not see how the original production can thus be accounted for. The 1 Evolution of Man, vol. ii. p. 391.

question is rather one of primary variation; which will be considered later on.

The difference between the germ and the sperm-cell, between the ovum and the spermatozoon, is enormous; and the process of their union, though a conjugation, is at all events inexplicable upon mere mechanical principles. To take a typical case of fecundation-that of Asterias glacialis,-when the ovum has arrived at maturity, a number of small vesicles "aggregate themselves into a single clear nucleus, which gradually travels towards the centre of the egg, &c. If, at this time, the spermatozoa [which are infinitesimally small as compared with the egg] are allowed to come in contact with the egg, their heads (they are composed of a nucleus or head and a mobile protoplasmic flagellum or tail), their heads soon become enveloped in the investing mucilaginous coat. A prominence pointing towards the nearest spermatozoon now arises from the superficial layer of protoplasm of the egg, and grows till it comes in contact with the spermatozoon." The spermatozoon then worms its way through the porous membrane, leaving its tail behind it. "At the moment of contact between the spermatozoon and the egg, the outermost layer of the protoplasm of the latter raises itself as a distinct membrane, which separates from the egg and prevents the entrance of other spermatozoa." The head having entered, forms a small pronucleus. "At whatever point of the egg the spermatozoon may have entered, it gradually travels towards the female pronucleus. The latter. . . remains motionless till the rays of the male pronucleus come in contact with it, after which its condition of repose is exchanged for one of activity, and it rapidly approaches the male pronucleus, . . . and eventually fuses with it."

Immediately after the fusion, what is called the eggcleavage or segmentation begins-first into two, then into four, eight, &c., successive parts. "At an earlier or later 1 1 Comparative Embryology, vol. i. p. 65.

stage," says Häckel, "the entire mass of cleavage-cells divides into two essentially different groups, which range themselves in two separate cell-strata-the primary germlayers." These germ-layers, as we shall see, contain the future, ay, and the past history of the growing organism.

When the segmentation is completed, "these segments usually form a wall one row of cells thick round a central cavity, which is known as the segmentation cavity, or the cavity of Von Baer. Such a sphere is known as a blastosphere. . . When the segmentation results in the formation of a blastosphere, one-half of the blastosphere may be pushed in towards the opposite half, and a GASTRULA be thus produced." This is called "invagination," from vagina, a sheath. Like the sheath formed by turning the finger of a glove into the hand of the glove, the cavity has two walls. These walls are the germinal layers out of which most of the animal organs are evolved. Of the two layers of which the gastrula is composed, "the inner one is known as the hypoblast, and the outer as the epiblast, while the pore leading into its cavity lined by the hypoblast is the blastopore." This blastopore when furnished with minute tentacles is really the animal's mouth.

Besides invagination, there is another way in which the cavity for ever after to be the alimentary tract—is formed: this is delamination. When the segmentation ends in a solid mass like a mulberry called a morula, either the surface of the morula may become depressed, and the dent may become a groove, and the groove may deepen into a cavity, which finally becomes a sheath;2 or else the solid mass of cells separate or delaminate straightway into two layers. Both methods occur, and both with numerous minor variations. Both, however, result in the differentiation of the epiblast and the hypoblast cells. In addition to these two layers there is a third or mid-layer-the mesoblast.

1 Evolution of Man, vol. ii. p. 186.

2 Ibid., vol. i. p. 186.

"The organs which may be regarded as mainly derived from the epiblast are-(1.) the skin; (2.) the nervous system; (3) the organs of special sense. Those from the mesoblast are- (1.) the general connective tissue and skeleton; (2.) the vascular system and body cavity; (3.) the muscular system; (4) the urinogenital system. Those from the hypoblast are the alimentary tract and its derivatives, &c."1

For further details there is no room here. Taken forwards and backwards, inductively and deductively, the lesson to be learnt from embryology is twofold: "From the one-celled organisation of the human egg and of the eggs of other animals, the conclusion directly follows, according to this fundamental law of biogeny, that all animals, including man, descend originally from a one-celled organism."2 "Ontogenesis, or the development of the individual, is a short and quick repetition of phylogenesis, or the development of the tribe to which it belongs, determined by the laws of inheritance and adaptation.”

Three branches of science contribute direct evidence to establish this latter principle: embryology, comparative anatomy, and palæontology. From embryology we learn that, at the commencement of his existence, man, like every other animal, descends from a single cell. By repeated division this cell is multiplied. A mulberry-germ is formed by the massing of these cells. The gastrula or cup-germ is developed either by invagination or by delamination. The cells of the inner lining are differentiated from those of the outer walls; and from these walls are formed a primitive intestine and a skin which is the potential source of movement, sensation, consciousness, and will.

From comparative embryology and comparative anatomy combined we gather that man, in some stage of his foetal development, passes through phases which agree in all substantials with the organisation of some of the 1 Balfour, ubi supra, vol. ii. p. 323.

2 Häckel, Evolution of Man, vol. i. p. 140.

simplest of both invertebrate and vertebrate groups of animals. "Each organism reproduces the variations inherited from all its ancestors at successive stages in its individual ontogeny which correspond with those at which the variations appeared in its ancestors."1 At one period of man's existence he possesses the "form-value" of the worm tribe, with a single intestinal tract having two orifices instead of the original blastopore. Later on, a spinal canal enclosed in a tube of skin is added; and man becomes a skull-less vertebrate like the lowest of this class, the living amphioxus or lancelet. Presently he passes into the icthyod stage. Two pairs of shapeless limbs appear, the homologues of dorsal and ventral fins. Gill-openings are completely formed, the first pair of gill-arches becoming afterwards upper and lower jaw. Seen in this particular stage, the human embryo, though well defined, is scarcely distinguishable from the embryo of such diverse animals as a bat, a tortoise, a chick, a dog, a sheep, &c.

Comparative anatomy, from beginning to end, is a continuous verification of the descent theory. It teaches that from fishes to amphibians, from amphibians to reptiles, from reptiles to birds, from birds to mammals, the passage is as gradual, even amongst living beings, in most classes, as is that from lemurs through the apes up

to man.

Palæontology, instructed by comparative anatomy, presents us with such volumes of facts testifying to the mutability and blood-relationship of different species, that it is almost impossible to select any one handful of these facts which shall be more weighty than another. The embryos of existing species again and again are found to bear the closest resemblance to ancient and extinct forms of life, although this resemblance is quite obscured in the adult form. Wide as the diversity seems at first sight between the flora and fauna of the past and present, 1 Balfour, Introduction.