Items related to A Contribution to the History of the Germlayers in...
Synopsis
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. 1887. Excerpt: ... III. THE ECTODERM AND ITS PRODUCTS. I. The Ectoderm. The origin of the first four ectodermic cells (micromeres) has been described under the head of cleavage and axial relations. By the addition of numerous other micromeres, arising, mainly, from the anterior and the lateral macromeres, a sort of blastodisc is gradually formed, centered at the upper pole of the egg. This blastodisc is not wholly ectodermic, for a few of its deeper cells, as we have seen, represent the earlier entoderm cells, as was first suggested by Bergh. The superficial, ectodermic portion of the blastodisc gives rise to the epidermal layer and its derivatives, the stomodaeum, sense-organs, etc. The ectoderm includes, in addition to the superficial portion of the blastodisc, all the teloblasts, except the two larger and and deeper ones, which represent mesoblasts. The grounds for regarding the eight smaller teloblasts as part of the ectoderm are the following: 1. They have at the outset a superficial position at the hind edge of the blastodisc. 2. Two of them give rise to the ventral nerve-cord. 3. In Lumbricus vide Wilson) they lie in, and plainly form a part of, the general ectoderm. 2. The Ventral Nerve-chain. In a preliminary paper (No. 16) I have already stated that the nerve-chain of Clepsine first appears in the form of two simple, unsegmented rows of cells; and, further, that each row is the product of a single cell, the neuroblast. At the time this fact was announced nothing of the kind was known in any other animal; and Nusbaum, the latest authority on Clepsine, had just arrived at an entirely different conclusion, and one altogether more in harmony with traditional views. A similar discovery has since been made by Wilson in Lumbricus, and, fortunately, the evidences in both ...
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