96 



PROP. AGASSIZ'S 



LECTURE XII. 



We have now to examine the highest group of 

 animals, which Naturalists have called Vertebrata 

 Upon this type more embryonic investigations 

 have been made than upon any other. It was, in- 

 deed, with this type that embryological studies 

 began when Dollinger, the Physiologist, traced 

 the growth of the young Chicken within the egg, 

 and for the first time showed how important for 

 physiological investigations it would be to under- 

 stand the manner in which organs were formed, 

 in order to arrive at more precise conclusions upon 

 their functions. And though Dollinger has writ- 

 ten nothing upon this subject himself, those who 

 are conversant with the history of Embryology, 

 acknowledge him as the first and most eminent 

 among those who have devoted themselves to 

 these investigations. Indeed, his pupils have, un- 

 der his directions, curried out his views, and de- 

 veloped the new science up to the point at which 

 it has arrived at the present day. 



The most eminent among Embryologists, in this 

 special department, (C. E. Von Baer) has been a 

 pupil of Dollinger ; and Pander and d' Alton, who 

 first published extensive researches upon the 

 growth of the germ within the egg of the Hen, 

 traced their investigations under Dollinger's im- 

 mediate superintendence. 



That the discovery of the unity of structure 

 among these highest animals, in their earliest con- 

 dition, has not excited more interest that the dis- 

 cove'ry of the egg among Mammalia has not been 

 more spoken of, and has not been considered as 

 one of the most brilliant points in the history of 

 Physiology is, perhaps, owing to the want of a 

 general understanding of these matters, and the 

 difficulty there was before of comparing, properly, 

 the egg in the various classes of Vertebrata, or of 

 introducing this subject before the public at large, 

 as it was done among professional men. But 

 really, it was in Physiology a great discovery 

 when it was ascertained that all Vertebrata, that 

 Fishes as well as Reptiles, as well as Birds, as well 

 as Mammalia, arose from eggs, which have one 

 and the same uniform structure in the beginning, 

 and proceed to produce animals, as widely differ- 

 ent as they are in their full-grown state, simply 

 by successive gradual metamorphoses ; and these 

 metamorphoses upon one and the same plan, ac- 

 cording to one and the same general progress. 



The unity of structure in Vertebrated animals 

 has been ascertained, has been understood, and 

 well understood, long before Embryology had ad- 

 ded anything to show how deep this unity of plan 

 was impressed upon that type. By the investiga= 



tions of Comparative Anatomy, it had been 

 tained that the external differences which charac- 

 terize the class of Fishes, that of Reptiles, that of 

 Birds, and that of Mammalia, were only modifica- 

 tions of one and the same structure that the head 

 of Fishes, for example, though apparently so dif- 

 ferent from that of Man, was made up of the same 

 bones, arranged in the same manner, only subdi- 

 vided into more distinct points of ossification, 

 with modified proportions, most of them remain- 

 ing moveable for life, but after all, arranged upon 

 the same uniform plan. 



It was especially in Osteology, that is, in the in- 

 vestigation of the bones, that this unity has been 

 traced at full length. It is, therefore, to that sub- 

 ject I would particularly call your attention with 

 reference to these general realizations, although 

 I cannot enter here into details of an illustration 

 of the facts. Anatomy has shown us the grada- 

 tion which exists among these animals to such a 

 degree of perfection, that in the leading and fun- 

 damental divisions there will be very little to im- 

 prove, though the details may be considerably im- 

 proved under the influence of embryolosical re- 

 searches. 



The order which is now assigned to the different 

 classes of Vertebrata is, to place Fishes as the low- 

 est class ; next Reptiles ; then Birds ; and Mam- 

 malia at the head. And this order of classifica- 

 tion, established from anatomical evidence, is 

 also confirmed by the differences which exist in 

 the mode of growth. Though starting from a 

 common structure in the primitive egg, the differ- 

 ent classes undergo metamorphoses,which proceed 

 in such a manner as to end in the establishment of 

 those final differences which characterize the struc- 

 ture of the different classes of Vertebrata. It is, 

 indeed, by the difference in the process of growth 

 that those differences which characterize the full 

 grown animals are brought about. 



It may therefore be said with perfect propriety, 

 that the higher Vertebrates undergo changes 

 through which, in different periods of their life, 

 they resemble the lower ones; that there is a pe- 

 riod when the young bird has the structure, not 

 only the form, but the structure, and even the fins, 

 which characterize the Fish. And of the young 

 Mammals the same may be said. There is a pe- 

 riod in the structure of the young Rabbit, (in 

 which the investigations have been traced more 

 extensively than in any other species,) when the 

 young Rabbit resembles so closely the Fish, that 

 it even has gills, living in a sac full of water 

 breathing as Fishes do. So that the resemblance 



