Chamisso on the Alternation of Generations in Salpa.
Trembley (see p. 57) had shown that Hydra, though an animal, multiplies by budding like a plant. He got indications, upon which he did not altogether rely, that it also propagated by eggs, and ten years later (1754) this supposition was confirmed by Roesel, who figured the egg, though he was unable to demonstrate that a young Hydra issues from it; subsequent inquiry has placed the fact beyond doubt. In 1819 Chamisso announced that Salpa, a well-known Tunicate which abounds at the surface of the sea, exhibits a regular alternation of the two modes of increase, the egg-producing form being succeeded by a budding form, the budding form by the egg-producing form, and so on indefinitely. Sars a few years later showed that the common jelly-fish Aurelia also propagates by eggs and buds alternately. Here the familiar swimming disks, which are of two sexes, produce eggs from which locomotive larva issue. The larva at length settles down and takes a Hydra-like form. It pushes upwards an ascending column, which divides transversely and forms a pile of slices, each destined to become a free, sexual Aurelia. The alternation of generations may be regarded as resulting from the introduction of budding into the early stage of a life-history which culminates in sexual reproduction, much as if a caterpillar were to divide repeatedly and form more caterpillars, each of which ultimately became a moth. The case which has been given as an illustration actually occurs in nature. A parasitic caterpillar, that of Encyrtus, divides while still an embryo, so that one egg produces several moths.[35] Many other cases of alternation have since been found among animals, and it seems to be the rule among plants.
Alternation of generations may be complicated by association with transformation, by the omission of stages usual in the class, and by budding-out from one part instead of from the whole body. In particular cases the complication becomes so great that biological language breaks down under it. Such terms as generation, individual, organ, larva, adult, cannot always be used consistently without either being strained or artificially limited.
Baer and the Development of Animals.
The curiosity of the ancient Greeks led them to look for the chick within the egg, and Aristotle mentions the beating of the heart as a thing which might be observed in a third-day embryo. After the revival of science Fabricius of Acquapendente figured the chief stages of development, from the first visible rudiments to the escape from the egg-shell. Harvey, the discoverer of the circulation, not only studied the developing chick, but took advantage of the rare opportunity of dissecting breeding does from the royal parks. His treatise on Generation is unfortunately impaired by Aristotelian philosophy, and some of the theories there set forth gave much trouble to Swammerdam. The oft-cited maxim "Omne vivum ex ovo" does not occur exactly in this form in Harvey's writings,[36] nor does it fairly state his own belief. Those who read his De Generatione will see that his knowledge was insufficient to justify so wide a generalisation; on this head it is enough to mention that he was persuaded of the production of insects without parents from putrefying matter.[37]
Malpighi was the first to apply the microscope to the embryonic chick. His figures are surprisingly full of interesting detail, and so far in advance of their age that they long failed to produce their due effect. On one point Malpighi unconsciously led naturalists astray for a hundred years or more. On examining a fowl's egg which he supposed to be unincubated, he discovered within it an early embryo. From this he concluded that the embryo pre-exists in the egg, like a plant-embryo in a seed. He mentions one circumstance which makes everything intelligible. The egg was examined in August, during a time of great heat, and the Italian summer no doubt started development, like the hot sand of Aden, in which Chinamen hatch their eggs. Swammerdam too enforced the same belief in pre-existing germs. From the fact that the butterfly can be revealed by opening the skin of a full-fed caterpillar he inferred (quite contrary to the opinion which he expresses elsewhere) that one animal had formed inside another. This led him to say that there is no such thing as generation in nature, but merely the expansion of germs which lie enclosed one within another. By his theory he explained how Levi could pay tribute to Melchizedek before he was born, and how the sin of Adam can be laid to the charge of all his posterity. The belief in the pre-existence of germs was first shaken by Caspar Wolff (see p. 81), who examined unincubated eggs but found no germ which could be detected by the histological methods then employed.
Swammerdam's Biblia Naturæ contains useful figures of early and late tadpoles; in particular, he describes a stage in which the body is entirely composed of rounded "lumps" or "granules," the cells of modern biology.
Early in the nineteenth century Pander and Baer, both of whom were pupils of Döllinger, a teacher of extraordinary influence, gave a new impetus to the study of development. Pander (1817-8) published an account of the early stages of the chick, illustrated by beautiful plates by D'Alton. Baer (1828-37) carried the work much further, not only greatly extending the knowledge of the developing chick, but discovering the mammalian ovum (1827), and announcing generalisations which down to 1859 were the most luminous that embryology had ever furnished; we may call him the founder of comparative embryology. He shows that development may supply decisive indications of the zoological position of animals; it teaches, for instance, that insects are of higher grade than arachnids or crustaceans, and that amphibians ought not to be united with reptiles. He describes the development of an animal as a process of differentiation, the general becoming special, and the homogeneous heterogeneous; differentiation is, he remarks, the law under which not only animals but solar systems develop. He maintains that the embryo, though gradually attaining complexity, makes no transition to a different type—e.g., the vertebrate is never in any stage anything but a vertebrate. All animals, he believes, are probably at first similar, and take the form of a hollow sphere (the gastræa of modern embryology). There are, he says, no new formations in nature; all is conversion. When he comes to speak of the pharyngeal clefts of mammals and birds, recently discovered by Rathke, he remarks that their correspondence with the gill-clefts of fishes is obvious. We wonder what is coming next, but our curiosity is not gratified by any memorable deduction. Neither here nor in his miscellanies (Reden), published nearly fifty years later, does he admit that mammals and birds can have descended from gill-breathing vertebrates. If we are inclined to hint that Baer, having gone so far, might well have gone a little farther, it is only fair to recollect that every leader in science is more or less open to the same reproach.