384 Julia B. Platt 
scattered peripheral cells of ectodermic origin, as in the course of 
the ramus ophthalmicus profundus trigemini, or of the nerve 
trochlearis, is in itself a demonstration of the exclusive assumption 
of nervous functions by the cells of the ectoderm. The formation 
of nerve fibres from such scattered cells — mesenchymal cells, save 
for their ectodermic origin — has been described by v. KUPFFER in 
Petromyzon (90, ’91), by Brarp (’87, ’88), FRoRIEP (91a) and 
myself 91) in the Selachii, and by myself in Necturus (96). 
It plainly shows that the wandering ectodermic (mesenchymal?) cell 
does not lose the ectodermic properties which distinguish it from 
neighboring cells of different origin. 
Perhaps no more striking instance of the independence of the 
ectoderm, as well as of the potential equivalence of all ectodermic 
cells can be found than that furnished by the regeneration of the 
lens, after extirpation in the Urodela, from the marginal epithelium 
of the retina. This replacement of a highly modified, and essentially 
non-nervous structure, originally derived from the external ectoderm, 
by cells belonging genetically to the infolded ectoderm of the central 
nervous system is described by Worrr (’94, 95) and confirmed by 
MÜLLER (96). 
Thus the phenomena of regeneration, as well as the normal 
development of ectodermic and endodermic tissues in the Vertebrata, 
speak strongly against the theory that each layer is capable of 
giving rise to all of the principal tissues. What we need is the 
explanation of the participation of more than one germ-layer in the 
formation of so-called mesodermic tissues, and of these tissues alone. 
A theoretical explanation lies near at hand. It is possible to 
suppose that, in the developing egg at the blastula stage, the 
protoplasm at opposite poles becomes differentiated in the direction 
of ectodermic or endodermie properties mutually exclusive of one 
another. At the same time an equatorial band of cells is deprived 
of these strictly ectodermic or endodermic properties (which we 
imagine concentrated in the cells at the opposite poles of the egg), 
but retains certain properties of which the polar cells have been 
deprived (ex. reproductive properties). The may suppose that there 
remain, however, a group of properties (ex. muscular) potentially 
common to all of the cells of the egg should circumstances call for 
their development, although usually developed in the band of equatorial 
cells alone, which in the infolded gastrula represent the mesoderm. 
Perhaps the theory would correspond more closely with observed 
