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The subject of comparative embryology grew rapidly during the nineteenth century as the field of comparative anatomy became better known, and when naturalists became interested in animals, not only as specific types, but also as the finished products of an intricate series of transformations.
It treats of the relations, anatomical, zoological, and geological, between the whole class of fishes, fossil and living, illustrated by numerous plates, while additional light is thrown on the whole by the revelations of embryology.
But we now know that embryology is even more direct in its proof that organic transformation is natural and real; while at the same time there is a completeness in the full series of developmental stages connecting the one-celled egg with the adult creature that must be forever lacking in the case of the fossil sequence of species.
They continue to be reproduced through the force of habit, and through indisposition to get out of any familiar groove of action until it becomes too unpleasant for us to remain in it any longer. It has long been felt that embryology and rudimentary structures indicated community of descent. Dr. For the fuller development of the foregoing, I must refer the reader to my work "Life and Habit."
In that interval of time the elucidation of the structure of the lower animals and plants had given rise to wholly new conceptions of their relations; histology and embryology, in the modern sense, had been created; physiology had been reconstituted; the facts of distribution, geological and geographical, had been prodigiously multiplied and reduced to order.
That Schwann should have gone to embryonic tissues for the establishment of his ideas was no doubt due very largely to the influence of the great Russian Karl Ernst von Baer, who about ten years earlier had published the first part of his celebrated work on embryology, and whose ideas were rapidly gaining ground, thanks largely to the advocacy of a few men, notably Johannes Muller, in Germany, and William B. Carpenter, in England, and to the fact that the improved microscope had made minute anatomy popular.
When we examine the embryology of the human nervous system, we must start from the important fact, which we have already seen, that the first structure of it in man and all the higher Vertebrates is the simple medullary tube, and that this separates from the outer germinal layer in the middle line of the sole-shaped embryonic shield.
The further development of the Ascidia in detail has no particular interest for us, and we will not go into it. The chief result that we obtain from its embryology is the complete agreement with that of the Amphioxus in the earliest and most important embryonic stages.
A fortunate accident had directed the attention of this able observer almost simultaneously to the embryology of the lowest vertebrate, the Amphioxus, and that of an invertebrate, the close affinity of which to the Amphioxus had been least suspected, the Ascidia.
But hereditary diseases and some other facts make me believe that the rule has a wider extension, and that when there is no apparent reason why a peculiarity should appear at any particular age, yet that it does tend to appear in the offspring at the same period at which it first appeared in the parent. I believe this rule to be of the highest importance in explaining the laws of embryology.
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