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Updated: May 14, 2025
All forms of animal life, from the coelenterates to the mammals, follow the same path in their embryological development as far as the gastrula stage, but here their paths widely diverge, those of each subkingdom going their own separate ways. We may infer, therefore, that during the pre-Cambrian periods organic evolution followed the lines thus dimly traced.
He published little, but he seems to have been the founder of modern embryological investigation, and to have initiated his two famous pupils, first Von Baer, and then Agassiz, into at least the rudiments of the doctrine of the correspondence between the stages of the development of the individual animal with that of its rank in the scale of being, and the succession in geological time of the forms and types to which the species belongs: a principle very fertile for scientific zoölogy in the hands of both these naturalists, and one of the foundations of that theory of evolution which the former, we believe, partially accepted, and the other wholly rejected.
The eggs are laid few in number, either singly or several together, on the under side of stones, chips or, as in the case of Isotoma Walkerii, under the bark of trees. They are round, transparent. The development of the embryo of Isotoma in general accords with that of the Phryganeidæ and suggests on embryological grounds the near relationship of the Thysanura to the Neuroptera.
The importance of embryological characters and of rudimentary organs in classification is intelligible, on the view that an arrangement is only so far natural as it is genealogical.
Unity of Type, Rudimentary Organs, Geographical Distribution, Relation of the extinct to the living Fauna and Flora, and mutual Relations of successive Groups of Fossil Forms. Light thrown on Embryological Development by Natural Selection. Why large Genera have more variable Species than small ones. Dr. Hooker on the Evidence afforded by the Vegetable Kingdom in favour of Creation by Variation.
The present is the child of the past, but we see that it is only in the present that such ancient groups as the colony of Hanseatic merchants in Old London have shown us what has been the ultimate significance of their embryological life.
It will produce a great effect both by the breadth of its general views and by the extreme sagacity of its special embryological observations. I have never believed that this illustrious man, who is also a man of warm heart, a noble soul, would accept the generous offers made to him from Paris.
The answer has been found in the immense array of embryological facts that investigators have verified and classified, that all tell the same story. It is, that birds have arisen by evolution from ancestors which were really as simple as the members of these lower classes.
This book is, perhaps, not as well known as Harvey's treatise demonstrating circulation of the blood, but it is an important work in the history of embryology and it occupies a prominent position in the body of English embryological literature. The famous frontispiece of the treatise shows Zeus holding an egg, from which issue animals of various kinds.
Let us now turn back to some of the earlier pages of the embryological record which we passed over in order that we might translate the later portions dealing with more familiar and intelligible structures like gills. Before the egg of the frog becomes an elliptical mass of cells, it is at one time a double-walled sac enclosing a central cavity; in this stage it is called a gastrula.
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