462 Mr. S. Hatta. Mesodermic Origin and the Fate of the 



placodes are purely nervous in nature and have no genetic relation to the 

 mesectoderm at all, although they are at certain stages in close contact with 

 the latter. In the course of the seventh day, therefore, before the first 

 appearance of the epibranchial placodes, the placode for the trigeminal 

 group is constricted off from the ectoderm. 



Finally, the origin of the branchiodermic cells in their first stage, of 

 which Schalk speaks, seems to be unintelligible. Judged from his text-fig. 12 

 and the accompanying statements, the cells are brought into their position 

 from the ectoderm not by cell-multiplication going on in this germinal layer 

 but simply by liberation of some of those composing the layer. If such 

 a case as given by Schalk really occurs, it might be looked upon as a case of 

 delamination. But. the occurrence of delamination, even for the formation 

 of the ventral parts of the mesoderm, as W. Scott (82) and, later, Mollier (06) 

 assume, or for the origin of the pericardium and endocardium, as asserted 

 by Shipley (87), has been disproved, and, according to my experience, occurs 

 in no case at least in the development of Petromyzon. In the series of 

 sections in my possession I find no similar case to that of Schalk, except 

 some sections frontally cut through the lower margin of the well-established 

 mesectoderm. 



According to the results obtained by myself the so-called mesectoderm is 

 not so peculiar a structure as it appeared to the previous observers, but it is 

 the mesoderm itself, a part of the somites or, as we may term them, the 

 sc! eromyotomes. It is represented at its first appearance, as observed in 

 the mandibular arch, by scattered free cells, which later coalesce for the most 

 part, to form a typical epithelium. 'In the postotic region the mesectoderm 

 is, on the contrary, from the first epithelial. 



In early stages there are found only two kinds of free cells : the blood 

 vascular cells and the mesectoderm cells. The cells of both kinds appear 

 almost at the same stage ; at about the fifth day from the fertilisation a few 

 vascular cells are to be observed in the space below the chorda, and between 

 the floor of the pharynx and the ectoderm which represents in these early 

 stages the ventral wall of the body.* 



On the contrary, the mesectodermic cells, the earlier traces of which are 

 seen already to the close of the fourth day, appear as a rule between the 

 ectoderm and the somatic plate of the mesoderm. When established the 

 mesectoderm is confined to the head and the branchial extent of the body. 



What interests us is that the mesectoderm is in the postotic region 

 nothing else than the ventro-lateral edge' of the scleromyotome which has 



* As to the full account on the characteristics of the blood vascular cells and on the 

 development of the vascular system I refer to my other papers (00, 07, 14a). 



