128 W. BALDWIN SPENCER. 
is formed the neural plate by a great proliferation of cells in 
the nervous layer; this plate becomes first of all grooved 
longitudinally by the primitive groove, and then in the well- 
known way the neural folds grow up dorsally, enclose the 
neural groove, and finally, after meeting each other, the neural 
canal. 
During this course of development the greatly thickened 
neural plate as it becomes transformed into the walls of the 
neural canal retains its primitive connection with the nervous 
layer of the epidermis as is well shown in figs. 7, 11, 16, &e.; 
in the first of these the neural folds have not yet fused dor- 
sally and the groove is wide open. 
The axolotl differs, however, in one respect from the frog, 
agreeing with the newt in having at first the epidermis of one 
cell thick, though after closure of the neural folds the two 
layers appear, and further development seems to be similar to 
that of Rana. 
This inner nervous layer is intimately connected with the 
development not only of the central nervous system, but also 
with that of the sense organs of the body, the auditory sac, 
the olfactory organ, the sense organs of the lateral line, and, 
most important of all, it is, as I hope to show, intimately related 
to the formation of the peripheral nervous system. 
Without taking the latter into account at the present 
moment there may be said to exist in the Amphibian embryo 
a complete superficially placed nervous sheath out of which 
not only the central nervous system, but all the sense organs 
of both head and trunk are formed and which gradually dis- 
appears as these reach their full development. 
The following notes refer to the origin of the cranial nerves, 
and I hope shortly to publish a complete account of these and of 
the spinal nerves. Owing tothe great difficulty in distinguish- 
ing the cells of the various germinal layers at early stages it 
is no easy matter to ascertain the manner in which the nerves 
develop and in the sections figured the mesoblast has been 
omitted for the sake of clearness. 
Before the closure of the neural canal no development of 
