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that the Articulata have their navel on their backs. Conversely, it is the characteristic of the evolutionary type of the Vertebrata that the position of the germ corresponds with the dorsal side of the animal. The formation of the dorsal groove, which subsequently closes to form the canal of the spinal cord, as it is gradually enveloped in a sheath growing from below, is followed by the formation of transverse plates, the pre-vertebral plates. The side plates lying outside of these grow towards the ventral side, and finally merge in the navel. The position of the actual vertebral column, consisting of separate vertebræ, is always originally occupied by a cartilaginous band, the notochord (chorda dorsalis), and, as from this axis, the germinal matter transforms itself into a tube above as well as below, into the spinal marrow with its sheath, and the ventral cavity with the intestinal canal,-Von Baer considered this mode of development as bi-symmetrical. The development of the Articulata he regards as simply symmetrical, and the development of the Molluscs he designated as massive. The justification of this is that the elongation produced by segmentation and the repetition of similar parts and sections of the body implicit in segmentation generally,—the metameric formation, as it is termed by Haeckel,—is totally foreign to the Molluscs.
We must now again repeat, that somewhat extensive. observations of the evolutionary forms of different animals lead at once to the belief that the embryos and evolutionary phases of higher animals are transiently more closely related to the complete and definitive conditions of the lower animal-forms, at least of the same
family; whence arose the fixed idea that the embryo of the higher animals passes through the forms of the lower animals. When natural philosophy, more especially in Germany, had elaborated this doctrine in a rather fantastical manner, and had proclaimed that Man was the sum of all animals, in structure, as well as in development," the doctrine," says Von Baer, "of the uniformity of individual metamorphosis with the vague metamorphoses of the whole animal kingdom necessarily acquired great weight, when, by Rathke's brilliant discovery, germinal fissures were demonstrated in the embryos of mammals and of birds, and the appropriate vessels were soon afterwards actually revealed."
The exaggerations and false inferences drawn from general analogies, and the vague ideas of types hovering above the whole, and regulating individual development, were wittily chastised by Von Baer.
"To convince ourselves that a doubt as to this doctrine is not utterly groundless, let us imagine that the birds. had studied the history of their development, and that it was they who now investigated the structure of the mature mammal and of man. Might not their physiological manuals teach as follows? These quadrupeds and bipeds have much embryonic resemblance, for their cranial bones are separate; like ourselves during the first four or five days of hatching, they are without a beak; their extremities are tolerably like each other, as are ours for about the same time; not a single true feather is to be found on their bodies, only thin feathershafts, so that, even in the nest, we are more advanced than they ever become; their bones are not very hard, and like ours, in our youth, contain no air at all; they
are utterly destitute of air-sacs, and their lungs, like ours in early infancy, are not full-grown; a crop is completely wanting; gullet and gizzard are, more or less, merged in a sac, all conditions very transitory in us, and, in most, the nails are awkwardly broad, as with us before breaking the shell; the bats, which appear the most perfect, are alone able to fly; not the others. And these mammals which, so long after birth, are unable to find their own food, and never rise from the ground, fancy themselves more highly organised than we?'"
Nevertheless, there remains the fact of the parallelism of individual development with the systematic series to which the individual belongs; and, among thousands of examples, we will select some of the most accessible and convincing. Polypes have always been placed systematically below the Medusa; in the development of many
Medusa (co np. Fig. 3, p. 43), a polype-like condition is interposed. The crinoid (Comatula), very common in the Mediterranean, is in its mature condition freely movable. This definitive development is, however, preceded by a sessile stage (Fig. 7), during which the body is attached to a stalk. During the larval period the animal resembles the permanently sessile genera, which, by all systematic rules, and by their geological position, occupy a lower rank in the series of echinoderms. The crabs, or anourous crustacea, are raised by sundry characteristics above their long-tailed congeners, among which is the fresh-water crayfish. In the course of development they pass through the long-tailed stage, as is shown in the larva (Fig. 8). It is by the abortion of the tail, which is employed by the long-tailed species as a natatory organ, that they become more fitted for running, and some of them for terrestrial life, as they are, in a measure, released from a burden.
One of the systematic series included in the Vertebrata, leads through the reptiles to the birds. Now, if, in the physiological reflections which Von Baer put into their beaks, the birds, as will appear later, were mistaken in boasting of their feathery garb in contrast to mammals and to man, they have, nevertheless, carried it a stage further than the reptiles, for the scale is the embryonic rudiment of the feather. Likewise, the tarso-metatarsal joint of the embryonic bird, with which we are already conversant (p. 9), and which is distinguished from the ankle-joint of mammals and of man, by its lying not between the leg and the tarsus, but in the tarsus itself, remains, as a definitive condition in the reptile, in the embryonic condition which in the bird it.
rapidly passes through. Although mammals are never actual fish, there is much that is fish-like in the embryonic phases of their organs; the embryonic fissures in the thorax correspond with the germinal branchial fissures; the formation of the brain may be traced to the complete brain of the lampreys and the sharks, &c.
In order to refute the doctrine that the embryo passes through the whole animal kingdom, Von Baer was content to prove that it never changes from one type to another. He repudiated the other, and more probable part of this theory, that is, that, at least within the types, the higher groups, in their embryonic phases, repeated the permanent forms of the lower ones, by terming it a question of mere analogies. The embryo, as it is gradually perfected by progressive histological and morphological differentiation, necessarily accords, in this respect, with less developed animals in proportion to its youth. "It is, therefore, very natural that the embryo of the mammal should be more like that of the fish, than the embryo of the fish is like the mammal. Now, if the fish be regarded merely as a less perfect mammal (and this is an unfounded hypothesis), the mammal must be considered as a more highly developed fish; and, in that case, it is quite logical to say that the embryo of the vertebrate animal is originally a fish." 1o
We have been somewhat faithless to our intention of confining ourselves in this chapter to facts only. The facts are too apt to provoke reflections, and we have, moreover, repeated these reflections merely as historical facts; we must now inquire whether they are really capable of satisfying us. I think not. It is by no means a merely histological and morphological differen