Page:Popular Science Monthly Volume 32.djvu/512

494 reptiles has long been in doubt. Professor Marsh has, however, discovered in Brontosaurus, one of the largest known Dinosaurs, two flat bones which he regards as clearly belonging to the sternum. They correspond to the immature stage of similar parts in birds.

Dr. Alexander Agassiz, in a study of the young stages of certain osseous fishes, shows that while the tail is a modified heterocercal one, it is for all that in complete accordance with embryonic growth and paleontological development; and, independently, Dr. John A. Ryder finds that "the median fins of fishes normally present five well-marked conditions of structure which correspond inexactly to as many stages of development, which, in typical fishes, succeed each other in the order of time."

Mr. James K. Thatcher, in a study of the "Median and Paired Fins, a Contribution to the History of Vertebrate Limbs," shows that "the limbs, with their girdles, were derived from a series of similar simple parallel rays, and that they were a specialization of the continuous lateral folds or fins evidenced in embryos, which were, with some probability, homologous with the lateral folds or metapleura of the adult Amphioxus."

A great amount of work has been done in making clear the earlier stages in the development of animals, and breaking down the hard and fast lines which were formerly supposed to exist between the larger divisions. Dr. C. S. Minot, in a series of papers on "Comparative Embryology," in referring to the work accomplished, says: "These researches have completely altered the whole science of comparative anatomy and animal morphology, by entirely upsetting a large part of Cuvier's classification and the idea of types upon which it was based, substituting the demonstration of the fundamental identity of plan and structure throughout the animal kingdom, from the sponges to man."

Professor C. O. Whitman, in describing a "rare form of the blastoderm of the chick, in which the primitive groove extended to the very margin of the blastoderm, terminating here in the marginal notch first observed by Pander," justly contends that, "in the origin of the embryo from a germ-ring by the coalescence of the two halves along the axial lines of the future animal, and, secondly, in the metameric division which followed in the wake of the concrescence," we have evidence of the annelidan origin of the vertebrates, since concrescence of the germ-bands is a well-established fact for both Chæetopods and leeches.