Articoli correlati a A Contribution to the Embryology, Life-History &...
Sinossi
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated.1882 Excerpt: ... a division of labor by exalting, in one group of cells, functions that are to a greater or less extent complementary to those exalted in other groups. A change of a similar nature occurs when an organism--Sacculina for instance,--removed from the struggle for existence by a parasitism that makes food and safty attainable without exertion, undergoes what is called retrogressive metamorphosis. The process of simplification brings the animal to a state in which the dissipation of force is minimized and the reproductive power enormously increased. It matters very little whether we call this process retrogressive or progressive, the result is evidently a great improvement in so far as the propagation of the species is concerned. The same result might be reached in a more direct way by arrest of development. In what other light can we regard Distomatous Sporocysts and Rediae? It is doubtless exceptional that development should stop short with the Gastrula stage, and that every cell should share the reproductive power: but since the number of cells in the active enjoyment of this power may vary in different animals from one to many, it is certainly not incredible that it should include all, particularly if the organism, halting at a very early stage, begins to reproduce under circumstances of an extremely favorable kind. When we reflect that in some animals (38, 50) the production of germ-cells is the peculiar work of the ectoderm; and further', that in the historical development of an animal not only may the same organ assume different functions at different times, but also the same function may shift from one set of cells to another (37), the fate of the peripheral cells of the Infusorigen ceases to be any serious objection to the view that they represent an ect...
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