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On the Mesodermic Origin and the Fate of the So-called 

 Mesectoderm in Petromyzon. 

 By S. Hatta (Sapporo, Japan). 



(Communicated by Prof. E. W. MacBride, F.R.S. Received December 10, 1914.) 



Introductory. 



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. 



1. Origin of the Mesectoderm. 

 The previous authors who deal with mesectoderm invariably assume its 

 ectodermic origin. The layer was so named by Piatt, because, in spite of 

 its supposed ectodermic derivation, it takes on in its further differentia- 

 tion features like a mesodermic structure. About the mode in which the 

 layer arises from its assumed mother-layer, positive evidence has not as yet 

 been given by any of the authors, except Schalk, who endeavours to make 



