92 ADAM SEDGWICK. 



space between the layers is never empty; it is always 

 traversed by strands of a pale tissue connecting the various 

 layers, and the growth which does take place at the places 

 mentioned is not a formation of cells, but of nuclei 

 which move away from their place of origin and 

 take up their position in this pale and at first 

 sparse reticulum which exists between the layer. As this 

 reticulum, which has always existed, becomes infested with 

 nuclei it increases in bulk, and forms the conspicuous reticulate 

 tissue which is by some authors called mesenchyme. The 

 primitive streak, the walls of the coelom, and the neural crest, 

 and, as Goronowitsch^ has shown, parts of the ectoderm, are 

 growing points where nuclei, not cells, are produced. These 

 facts I described long ago in the development of Peripatus, 

 and it is the recognition of the same processes taking place in 

 the Vertebrata in an even more conspicuous manner that has 

 induced me to again call attention to their importance.^ 



The Origin of Nerve-trunks and the Fate of the 



Neural Crest. 

 If there is one point more than another on which the cellular 

 theory of development has led anatomists completely astray, it 

 is upon this one. We may take it that the new views upon the 

 origin of the peripheral nerves began with Balfour's discovery 

 of the structure which is generally called the nerve crest. 

 Before that discovery nerves were supposed to develop in situ 

 in the mesoderm ; after it, there were two principal views as 

 to the origin and growth of nerves : one of these was that cells 

 of the central organ grew outwards as strings to the periphery ; 

 while, according to the other, nerve-fibres are the elongated 



1 ' Morpholog. Jahrb.,' Bd. xx, 1893. 



^ At the same time Peripatus shows certain features more clearly than 

 the Vertebrate ; I would refer especially to figs. 24 d aud 26 d ou pi. v of 

 my Monograph, in which, while the so-called ectoderm and endoderm are 

 obviously parts of the same layer, or tissue : they are separated by a region in 

 which the vacuoles are larger, the protoplasmic strands less numerous, and 

 nuclei are conspicuous by their scarcity. 



