ON THE CELLULAR THEORY OP DEVELOPMENT. 93 



processes of cells either of the central organ or of the ganglia. 

 Both these views are erroneous; and if both were not inspired 

 by the cellular theory of development, they were both promul- 

 gated at a time when that theory was at its zenith. The earlier 

 view, that nerves were developed in situ from the mesoderm, 

 was much nearer the truth. 



The nerve crest does not, as was first stated by Balfour and 

 afterwards by all authors on the development of nerves, give 

 rise exclusively, or even principally, to nerves and ganglia. It 

 gives rise to nuclei which spread out in, and add to the meso- 

 blastic reticulum, which at all times, i.e. from the very begin- 

 ning, exists between the layers, and to nuclei which become 

 the nuclei of the rudiments of nerve ganglia. The nerves are 

 developments of the reticulum ; they are elongated strands of 

 the pale substance composing the reticulum, with some of its 

 nuclei ; and their free ends branch out into the fibres of the 

 reticulum, and are added to by the latter falling into the line 

 of the growing nerve. Neither they nor the ganglia appear 

 until the nerve crest is breaking up. The reticulum further 

 gives rise certainly to smooth muscular fibres, connective 

 tissues, and blood-vessels, and probably also to striated 

 muscle. It is also continuous with all the so-called epithelial 

 tissues of the embryo; indeed this latter substance is to 

 be regarded as consisting only of one or more layers 

 of nuclei embedded in the outer part of the reticulum, 

 which is rather denser than elsewhere in correspond- 

 ence with the greater density of the nuclei. Nerves 

 are a gathering up, so to speak, of the strands of the reticulum 

 into bundles, and are formed in that way; or, to put the matter 

 in another way, nerves are a special development of the reti- 

 culum along certain lines. These special developments are 

 generally marked by an increase in the number of nuclei, such 

 increase being particularly great in the neighbourhood of the 

 ganglia. 



To sum up the matter, the nervous and muscular tissues are, 

 as they were in Peripatus (see my Monograph, p. 131), 

 special developments of the same primitive reticulum, a com- 



