494 THE POPULAR SCIENCE MONTHLY. 



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,f 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 J 

 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 Avere a specialization of the con- 

 tinuous 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 develojDment 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 blas- 

 toderm 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 concres- 

 cence of the germ-bands is a well-established fact for both choeto- 

 pods and leeches. 



* *' American Journal of Science and Arts," vol. xix, p. 395. 



f " ProceedingH of the American Academy of Arts and Sciences," vol. xiii, p. 117. 

 X "American Naturalist," vol. xix, p. 90. 



* " Transactions of the Connecticut Academy of Arts and Sciences," vol. iii, p. 281. 

 I Ibid., vol. xiv, p. 96. 



^ " Proceedings of the British Society of Natural History," vol. xxii, p. 178. 



