458 Mr. S. Hatta, Mesodermic Origin and the Fate of the 



intelligible, by means of figures and descriptions, the precise mode in winch 

 it originates. 



According to v. Kupffer and Koltzoff, the mesectoderm is formed of cells 

 liberated partly from the medullary cord, but in the main from the ectoderm 

 forming the lateral walls of the head and of the branchial section of the 

 body. The cells of the branchial region are regarded by them as being 

 proliferated from the ganglionic placodes. These cells, whether medullary 

 or ectodermic in origin, are classed together by the authors and designated 

 as ectodermal, because the cells from the two sources become so much 

 intermingled as to be indistinguishable, when once they have left their 

 mother-layers. 



But the cells of the mesectoderm become differentiated, as their later 

 history shows, in two directions : viz. into the cephalic nerves with their 

 internal ganglia on the one hand, and into the tissues which give rise to 

 the cartilaginous branchial basket and the connective tissue on the other. 

 For this reason probably the mesectoderm has been designated either as 

 neurodermis or as branchiodermis, until Koltzoff (02) identified it with 

 the corresponding structure found by Piatt in Necturus lateralis. Neither 

 v. Kupffer nor Koltzoff were able to distinguish in the mesectoderm the 

 nerve cells from the other elements, but each states that both the nerves and 

 connective tissue are derived from one and the same source. 



Koltzoff distinguishes in the mesectoderm the dorsal division, which is found 

 above and within the cephalic ganglia, and the ventral division, which extends 

 below them towards the ventral surface. This attempt amounts to nothing 

 and is founded only on histological distinction ; the dorsal division is a 

 network of the strings caused by the connection of the cells with one 

 another by their protoplasmic processes, while the ventral division of the 

 layer consists of a typical epithelium of columnar cells. But this histo- 

 logical difference is a temporary one, and does not indicate, as Koltzoff 

 believes, a differentiation of the nerve cells from the other elements of the 

 mesectoderm. 



The principal point in which the results by Schalk differ from those of the 

 authors above mentioned consists in the origin of the mesectodermal cells, 

 which, according to him, is confined absolutely to the ectoderm ; he denies 

 positively that any of these cells have a medullary origin. He says, however, 

 nothing definite about the nerve cells or the mesoderm somites, except that 

 the sclerotomes give rise to the trabecule and parachordals. 



If I understand Schalk correctly, there are two phases in the liberation of 

 the mesectodermal cells from the ectoderm. In embryos about ten days old 

 selected from his material, the heads of which have just begun to be raised 



