MORPHOLOGICAL STUDIES. 
167 
some little distance from the infolding neural plate it attains 
its maximum thickness (leaving the neural plate itself out of 
question as part of the epiblast). Beyond this point it abruptly 
becomes one- layered again, and remains one-layered till it ends 
also abruptly in the neural plate. The region of this one- 
layered epiblast is that from which the ganglionic Anlage has 
been cut out. The point of maximum thickness is that portion 
of epiblast which has just failed to take any share in the forma- 
tion of the ganglion. This point was one which gave me a 
good deal of trouble in the course of the researches, but the 
explanation of it gave the key to the origin of the ganglionic 
Anlagen. In fact the first rudiments of the ganglia are formed 
from the deeper layers of the epiblast just outside the limits of 
the neural plate. 
These stages in the formation of the spinal ganglia have 
never yet been seen or figured by any observer. 
The involution of the neural plate now begins to take place 
very rapidly (fig. 15). Along with it the ganglionic Anlagen 
get carried upwards. It seems as though they had not time to 
get out of the way of the infolding process, and in missing the 
chance to get out before the involution begins they are bound, 
on account of pure mechanical processes — the explanation and 
description of which I leave over to others — to follow the 
neural plate, and thus they come to a somewhat abnormal 
position at and between the dorsal lips of the neural plate. 
The steps of this process are shown successively in figs. 14, 15, 
5, 9, 21, 32. 
Some figures of the head region (Nos. 38 — 42) are given 
under high magnification, and tell their own tale in justification 
of my statements of the very early appearance of the ganglia as 
epiblastic proliferations and their independence of the neural 
plate. They lie close to the latter, but can no more be 
regarded as outgrowths of it than any other two organs which 
lie close to each other in development can be considered, for 
that reason, as derivatives one of the other. I have nearly 
always been able, after the first traces of the ganglia were 
visible, to distinguish the lateral limits of the neural plate, 
