258 ADAM SEDGWICK. 



3. The view of His/ which was previously held by Bidder. 

 According to this view the nerve-fibres are the elongated pro- 

 cesses of cells. The anterior roots are derived from non-cel- 

 lular outgrowths of the spinal cord, consisting of the elongated 

 processes of the nerve-cells of the central organ. The fibres 

 of the posterior roots, on the other hand, are the elongated 

 processes of the ganglion-cells of the ganglion on the posterior 

 roots. Processes of these cells grow out to the periphery and 

 inwards to the centre. 



Balfour expressed on a priori grounds a strong preference 

 for the view of Hensen, but rejected it on the ground that 

 there was no evidence for the connection which it demanded. 

 Now, however, we know that in many types the segmentation 

 of the ovum does not bring about a complete separation of the 

 cells of the ovum.^ 



There is no such separation inPeripatus; and in many 

 Arthropod a — if not in all — it is known not to take place. It 

 does not take place in Elasmobranchs, as I can certify from my 

 own observations; but for a summary of the facts and a dis- 

 cussion of the whole question I must refer the reader to my 

 monograph already quoted.^ 



If the segmentation of the ovum does not bring about a 

 complete separation of the cells of the germ, as it was formerly 

 supposed to do, then the connections required by Hensen's 

 theory exist. 



Turning to the special case before us of the Vertebrata, I have 

 in the present paper dwelt upon the fact (see above, p. 256) 

 that the cells of the young embryo (subsequent to cleavage) are 

 connected by delicate processes, and that these processes are 

 often extremely fine, and unite together into networks below 

 the epithelial arrangement of the protoplasm which is charac- 

 teristic of the surfaces. This network is sometimes of a very 



1 ' AnatomiscLer Anzeiger,' vol. iii, p. 500. 



* See Self, "Monograph on the Development of Peripatus capensis," 

 in ' Studies from the Morphological Laboratory of the University of Gam- 

 bridge,' 1889, pp. 47—50, and pp. 130, 131. 



' Loc. cit., pp. 99—106. 



