134 W. BALDWIN SPENCER. 
rounds the whole body. In Elasmobranchs the whole epiblast 
does not divide into two layers, and hence the nerves must be 
developed in some different way. After formation of the neural 
canal the nervous matter of its walls grows very rapidly, and 
gives off on each side the neural ridge, from which as lateral 
outgrowths the nerves are formed by cell proliferation, the 
parts of the ridge not used in development of the nerves disap- 
pearing, just as in the Amphibia, after formation of the nerves 
and sense organs, the remainder of the nervous layer gradually 
atrophies, and finally disappears as a distinct structure. 
This will, at all events, explain the fact of there being a con- 
tinuous neural ridge along the dorsal surface of the central 
nervous system, the intermediate parts of which disappear after 
formation of the nerves. 
The proliferation of nerves in Amphibia shows a remarkable 
resemblance to that of the lateral and pedal nerve-cords which 
has recently been described by Kowalevsky as occurring in 
Chiton.! Fig. 25 is copied from one of his figures (fig. 60) 
and shows the thick epiblast on the ventral surface giving rise 
by simple proliferation to the nerves, and when compared with 
figs. 16 and 19 of Amphibian sections the process will be seen 
to be an essentially similar one in the two cases. 
It is interesting further to observe that a neural sheath is 
actually found existing throughout life in some of the inverte- 
brates, in which sheath greater or less differentiation for the 
formation of nerves takes place. 
Thus in some Ceelenterates a rudimentary and ill-defined 
sheath may be present; in the schizo- and palzo-Nemertines 
one is well developed and in the latter les close to the epiblast, 
whilst long lateral nerve-cords are developed from the sheath, 
though they retain their connection throughout life, while 
finally, Professor Marshall has suggested that the Echinoder- 
mata may be supposed to have had a nerve-sheath immediately 
below the epidermis surrounding the whole body, and out of 
which both the nerve-cords of crinoids may be formed, the 
1 Memoire No. 5, ‘Annales du Musée Histoire Naturelle de Marseille,’ 
** Embryogenie du Chiton Polii.” 
