On the mesodermic origin and the fate of the so-called mesectoderm in petromyzon

Abstract
About 20 years ago v. Kupffer (85) described in the embryos of Petromyzon an epithelial structure extending, between the ectoderm and the somatic plate of the mesoderm, from the head to the posterior boundary of the branchial region, and described it under the name of the neurodermis; subsequently, he bestowed on it the name branchiodermis. Seventeen years later the same structure was again discovered by Koltzoff (02), who identified it with the mesectoderm which was described by Miss Piatt (94) in Necturus embryos. Subsequently, so far as Petromyzon is concerned, nothing was published until last year, when a paper by Sehalk (13) appeared, although the corresponding layer of cells was described by A. Dohrn (02) in Selachii and by Brauer (04) in Gymnophiona. For a long time the origin and fate of the layer in question engaged my attention. Last summer I was able to re-examine my sections and to confirm observations which I had previously published in a paper entitled “Die Bildungsweise und erste Differenzierung des Mesoderms beim Neunauge ( Lampetra mitsukurii , Hatta),” in which, the origin and differentiation of the so-called mesectoderm are described and illustrated by a series of microphotographs. To my regret the paper, which was ready for press when the great war broke out, could not be sent to the editor of a certain scientific journal in Belgium, who had promised to publish it in his journal. The present note is an attempt to communicate some of the principal points of that paper which relate to the mesectoderm. The other organs dealt with in the above-mentioned paper have already been described in preliminary notes or in my previous papers.

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