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, Ramon y Cajal, and others had not 

 brought into prominence the neurone theory, i.e. 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 



2 H 



