Twelve lectures on comparative embryology : delivered before the Lowell Institute, in Boston, December and January, 1848-9 / by Louis Agassiz ... Phonographic report, by James W. Stone ... Originally reported and published in the Boston Daily Evening Traveller.

  • Louis Agassiz
Date:
1849
Catalogue details

Licence: Public Domain Mark

Credit: Twelve lectures on comparative embryology : delivered before the Lowell Institute, in Boston, December and January, 1848-9 / by Louis Agassiz ... Phonographic report, by James W. Stone ... Originally reported and published in the Boston Daily Evening Traveller. Source: Wellcome Collection.

    12/116 page 6
    facts could be acquired. The knowledge of a few fish, enabled him to compare the whole class of fishes with reptiles; a knowledge of a few rep- tiles, enabled him to institute extensive compari- sons between reptiles and birds; and again, be- tween these and mammalia; andto find that all these animals agree in certain respects. And how- ever many have been examined since,—and three or four times more have been examined than the number which Cuvier had known when he laid out his classifieation—however many have been studied since, they have all been found to agree in these essential particulars. So that it is now plain, that structure is the principle upon which animals can be most satisfactorily classified. And as I shall often have occasion to refer to this classification, let me at once, in a few words, indicate which great divisions Cuvier introduced into his animal king- dom. ° All the animals which I have mentioned, Fishes, Reptiles, Birds, and Mammalia, are combined to- gether, because they have a series of backbones, called vertebrae, by anatomists; and hence the name of vertebrated animals. They agree in the general structure of their brain ; they agree in the general arrangement of the fleshy parts, and in the genera! arrangement of the organs of life— as of the organs of respiration, the heart, the ali- mentary canal, and so on. Another group,which was established on the same principle, is that to which we may refer worms, insects, crabs and lobsters—all animals whose bo- dies are divided into a series of moveable rings,— joints, which surround the body and enclose the soft parts; and which are provided with move- ablelegs,and in some, even in addition to these legs. also with wings. All these animals have a most remarkable arrangement of the nervous sys- tem; there being a series of swellings of nervous substance placed, one in each of the rings, and connected together by double threads; so that the nervous system is all contained in one cavity, not only the general arangement of parts,but this most important organ of life is also different from that of vertebrates. The next great group is that of Mollusca, contain- ing cuttle-fish, snails, slugs, clams, and oysters,— all those animals which we generally call shell-fish —those which are provided with hard structures— the body being soft and generally surrounded by a great quantity of mucosity ; the nervous system consisting simply of a circle surrounding the ali- mentary tube, with a swelling above the intestine, and another below, from which all the nervous threads arise, which are diffused into all parts of the body. In these three groups of the animal kingdom, all parts are in pairs, placed on two sides of the longitudinal axis. In all of these there is an ante- rior and posterior part; two sides, a right side and a left side; and they have a back part and a lower part; they are, in fact, symmetrical. But there is another group, in which there is @ different arrangement. The mouth is in the cen- tre of acireular body; and from this mouth, the organs are placed like rays, diverging in all direc tions. Here we have no right, and no left side, no anterior and no posterior extremity. The body is star-shaped ; and the nervous system has thesame general structure, consisting of an horizontal ring around the entrance of the alimentary tube, and has no longer an upper and a lower swelling, as in Mollusca. There have been a few modifications made in the details of this arrangement as proposed by Cuvier. Some of the animals placed among the mollusca, Barnacles are one ef this group; a very remarkable © family, from.the numerous shells around the body. - Without knowing certainly, he had placed them among the mollusca; but on examination, it was found that their nervous system consisted of swel- lings, and that their’bodies were divided into joints —and an additional evidence was obtained from a knowledge of their young, which were found to resemble, in the earlier stage, much more the crus- tacea than the mollusca; and indeed, that they were crustacea, and assumed this covering only at a later epoch. However important these anatomical researches have been,it is nevertheless my belief thatin this line of investigation we have gained all the important information that we can gain ; and that we have to run new tracks in order to improve our natural method,—that we must even give up this funda- mental principle, as the ruling principle, if we will make further advance in this science. And my reason is this: The minute investigations which are now making in the anatomy of animals,are bring- ing forward such differences between them, that we have no principle by which we can appreciate their value. And if we consider every difference in structure as sufficient to separate animals, the time would come when we should formas many groups —as many divisions—as there would be smaller groups in the animal kingdom, as it can be shown that even genera differ anatomically among them- selves. If Iam not entirely mistaken, these new investi- gations, this new information, must be derived from embryological data. It is to the study of young animals—it isto the investigation of the formation of the germ within the egg, that we must appeal for a ruling prinviple to ascertain the real, natural position of the subdivisions of the mi- nor groupsin theanimalkingdom. I acknowledge that the great divisions will alwaysstand on the an- atomical structure. But the subdivisions of the classes cannot rest upon anatomical investigation . and ifI do not fail in my endeavors, I hope to show it to you satisfactorily. This new step is a natural consequence of the natural progress and state of Investigations have recently been carried on,
    digitised image 1
    digitised image 2
    digitised image 3