The Development of the Cartilaginous Skull ete. in Necturus. 383 
It is possible to explain the conflicting facts in accordance with 
the view advocated by v. KÖLLIKER (loc. eit.), and to maintain that 
the protoplasmic differentiation in embryonic tissues is so slight that 
any cell may, at the need of the organism, replace another. Thus 
the ultimate fate of any cell would depend chiefly on its position in 
relation to surrounding cells. Such a supposition makes early em- 
bryonic development the result of mechanical rather than chemical 
changes in the growing organism, and tends in the direction of the 
experimental work of WıLson, Morean, Roux, and others, by whom 
it has been shown that cells separated in early cleavage stages may 
each develop into an entire organism, when under normal conditions, 
each such cell gives rise but to a definite part of the individual 
formed from the whole egg. 
BARFURTH (95, page 349) notes, however, in the report on 
Regeneration, that the experiments of DrıEscH (95) on Echino- 
derm larvae lead him to conclude that the ectoderm is not capable 
of reproducing the lost cells from which the mesenchyma, and 
digestive tract are formed. BarrurtrH’s own experimental study of 
the regeneration of the germ-layers in the Amphibia lead to the 
same result as regards the independence and specialization of the 
germ-layers, which is further confirmed by Mor@an (95). BARFURTH 
also cites the experiments of Barra (95) on the implantation of 
bone as tending to demonstrate the independence of the layers. We 
stand before the fact, BARFURTH says (page 374), that on the one hand 
the entoderm, if you choose, cannot replace the ectoderm in regene- 
rative development, while on the other hand both layers in typical 
development may -produce epithelia as well as glands, muscles, and 
other tissues, and the question consequently arises, whether, for 
example, a smooth muscle fibre derived from the mesenchyma is 
actually in every respect similar to the smooth muscle fibre produced 
by the ectoderm. 
Although the mesoderm was once associated with the ectoderm 
as a source of nervous tissue, it is at present seldom, if ever, affirmed 
that the mesoderm takes part in the formation of nerves or ganglia, 
unless, with KASTSCHENKO (loc. cit.) and GORONOWITSCH ('92), we 
include in the mesoderm, i. e. mesenchyma, all wandering cells of 
whatever origin, that chance, even temporarily, to lose their epithelial 
connection. 
The normal development of the peripheral nervous system in 
lower Vertebrates, with the attendant growth of nerve fibres from 
