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. The Cell Theory.