NOTES ON ELASMOBRANCH DEVELOPMENT. 583 
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 in Peripatus; and in many 
Arthropoda—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. 581) 
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 ¢ Anatomischer Anzeiger,’ vol. iii, p. 500. 
2 See Self, “‘ Monograph on the Development of Peripatus capensis,” 
in ‘Studies from the Morphological Laboratory of the University of Cam- 
bridge,’ 1889, pp. 47—50, and pp. 130, 181. 
3 Loe. cit.,-pp. 99—106. 
