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
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.
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![swellings; next, a great spacewithout lateral nerves; and then, a space with swellings at equal distances, corresponding to the abdomen. Remember now the arrangement of rings and legs in the Caterpil- lar and in the Grasshopper, [Plate XV], You will see that the arrangement of the external parts agrees with that of the nervous system. The head consists of one undivided mass [Plate XV. fig. A] There are three pairs of horny claws in the Caterpillars (Plates 1X, X, and XI,) and three rings to the chest in the Insects proper, (Plate XV, figs. B, C, D) receiving nerves from the concentrated swellings of the anterior part of the body. Then, there is a region from which no nerves are deriv- ed; and a region from which four pair of sucker- like legs are produced, answering to the regionin which these four swellings remain equally distant; and then another region, of two rings without; and another, last. with suctorial legs, which corres- ponds to the large terminal nervous swelling. It is a question which it is not possible to solve now, and which it will be very difficult to solve, if it can be solved at all, whether the larger terminal swelling of nervous matter consisted originally of one nervous mass; and whether the anterior ce- phalic ganglion consisted also, primitively, of one nervous mass. That it consists of two now, is shown here, [Plate XI. fig. A] by the entire disap- pearance of the first small ganglion. But there may be other changes in the structure of the ner- vous system, taking place previous to the full growth of the Caterpillar. And this remains for the present undecided. But,so muchis shown as to prove that the nervous system is equally dis- tributed in the solid rings, and they will gradually combine in such a manner as to present arrange- ments answering to the changes which take place in the external form. There is one mass more, properly belonging tc the head, another mass more concentrated, belonging to the chest, and another mass remaining stationary and belonging to the abdomen. We now can, withsthese facts, arrive at another general conclusion, viz.: that wherever among ar- ticulated animals, among Insects, we find the ner- vous system constituted of equally distributed ner- vous swellings, such animals are lower than those in which several swellings unite together to form few masses. Now, in this respect, what do we ob- serve in the different classes compared together ? I now no longer compare the same animal in its different stages of growth, but different classes of Articulata with each other. What do we observe in comparing Insects with Worms,and Worms with Crustacea? All worms have equal rings and very numerous joints ; and joints which are never com- bined so as to form regions distinct from each other. There is never a distinct thorax or abdo- meninany Worm. So that, from what we have learned, we know that the lower position assigned, for many and all sorts of good reasons, to worms, is the proper position which they must preserve; and where a nervous system has been observed among them, it agrees with the condition of that system in Caterpillars, rather than with that of the later metamorphoses. The question reniains be- tween Crustacea and Insects. What is the condi- tion of the nervous system in Crustacea? The nervous system occurs in various conditions there. In the lower Crustacea,the swellings being scatter- ed all along the body, one to each ring—a condi- tion which we observe in the earlier stages of growth in the Caterpillar. Next, we have other Crustacea in which the nervous swellings contract and combine together, nearer and nearer. But in them, strange to say, there is only one point of concentration. And then there are Crustacea, as the Crabs, in which the nervous system is con- tracted into one single, central mass. And the question is, what shall we consider superior ?—an arrangement which -gives rise to several distinct centres, and corresponds to distinct regions of the body, (as in Insects, Plate XII. fig. F, and Plate XIV, fig. A) a head,with a central nervous swelling of a peculiar kind ; a chest, with a nervous mass of a peculiar kind, sending its thread to the legs of that region; and another posterior combination of nervous swellings, corresponding to the other re- gion, called abdomen, and sending nerves to its part? It seems to me that we cannot remain doubtful. We cannot fully derive this conclusion from direct investigations, as we have not, in any instance, @ case to settle it by direct comparison ; but we may say,that in Crustacea we have concentrated unifor- mity; while in Insects in their perfect condition, we have concentrated diversity. And, if we are al- lowed to compare the one with the other, I would incline to the opinion that concentrated diversity, with prevailing influences over peculiar functions of the life of the different centres, is a condition of structure which stands higher than concentrated uniformity,in which we have only one centre. We have all the primitive diversity reduced to one centre, which does not acquire any distinct influ- ence upon different parts. The alimentary canal undergoes corresponding metamorphoses. Here is the straight tube (Plate XIII, fig. A) of the digestive canal of a Caterpillar. It is very wide in comparison to its length, and ca- pable of digesting an immense mass of food, com- paratively to the size of the animal. In its earlier condition, it is provided with an apparatus which disappears afterward. There are considerable sali- vary glands in the anterior portion of the alimen- tary canal, which disappear in the pupa state and do not exist in the perfect insect. These figures (Plate XIII) must impress you as very singular.— No animal has mbre curious organs than this In- sect. The liver, or hepatic glands, and the salivary glands are massive organs in other animals. Here, they are slender tubes, and form little winding branches on the sides of the alimentary tube. In- eed, all glandular organs in Insects have such a](https://iiif.wellcomecollection.org/image/b33278982_0064.jp2/full/800%2C/0/default.jpg)
