THE PRINCIPLES OF EMBRYOLOGY 465 
host is in the adult composed of a number of double elements, a 
nerve-cell element, and an epithelial element, such as muscle-cell, 
gland-cell, etc., connected together by nerves; and if such connection 
is always present as we pass from the adult to the embryo, if there is 
no period when, for example, the neural element exists alone free from 
the muscle-cell, no period when the two can be seen to come together 
and join, then it follows that when the single-layered blastula 
stage is reached, muscle-cell and nerve-cell must have fused together 
to form a neuro-muscular cell. Similarly with all the other neuro- 
epithelial organs; however far apart their two components may be 
in the adult, they must come together and fuse in the embryo to 
form a neuro-epithelial element. 
The close connection between muscle and nerve which has always 
been recognized by physiologists, together with the origin of muscle 
from a myo-epithelial cell in Hydra and other Ccelenterata, led the 
older physiologists to accept thoroughly Hensen’s views of the neuro: 
epithelial origin of all tissues connected with the central nervous 
system. Of late years this conception has been largely given up 
owing to the statement of His that the nervous system arises from a 
number of neuroblasts, which are entirely separate cells, and have at 
first no connection with muscle-cells or any peripheral epithelial 
cells, but subsequently, by the outgrowing of an axial fibre, find 
their way to the muscle, etc., and connect with it. I do not think 
that His’ statement by itself would have induced any physiologist to 
give up the conception of the intimate connection of muscle and 
nerve, if the work of Golgi, Ramdn y Cajal, and others had not 
brought into prominence the neurone theory, 7.c. that each element 
of the central nervous system is an independent element, without 
real connection with any other element and capable of influencing 
other cells by contact only. These two statements, emanating as they 
did from embryological and anatomical studies respectively, have 
done much to put into the background Hensen’s conceptions of the 
syncytial nature of the motor, neural, and sensory elements, which 
make up the master-tissues of the body, and have led to the view 
that all the elements of the body are alike, in so far as they are 
formed of separate cells each leading an independent existence, 
without any real intimate connection with each other. 
The further progress of investigation is, it seems to me, bringing 
us back to the older conception, for not only has the neuroblast theory 
2H 
