BRANCHIAL SENSE ORGANS OP ICHTHYOPSIDA. 175 



account is in accordance with my own researches, and contains some 

 additions to the accounts given by my predecessors.) 



The nerve grows outwards and downwards from the neural ridge 

 towards the lateral surface of the head. In its course it lies directly 

 under, but unconnected "with, the epiblast. In the case of those nerves 

 which are connected with gill-clefts, and are therefore typical, the 

 nerve lies just over the cleft (fig. 50). All this is well known, and 

 has been described by Balfour, Marshall, Van Wijhe, &c. 



The subsequent events are as follows •} 



1. When the nerve reaches the level of the notochord, or a little 

 below that level, it fuses with the epiblast (fig. 34). 



2. Part of the nerve, however, passes on to the lateral muscle-plates 

 of the segment (figs. 34, 50). 



3. At the point of fusion mentioned in 1. a local thickening of 

 epiblast has previously taken place (fig. 14). 



4. After the fusion has taken place a proliferation of some of the 

 cells composing the thickening ensues. The proliferated cells form a 

 mass of actively dividing elements still connected with the skin and 

 fused with the dorsal root (fig. 16). 



5. This mass of cells is the rudiment of the ganglion of the dorsal root, 

 and externally to it is situated the rudiment of the primitive branchial 

 sense organ of that root (figs. 12 and 13). 



6. For some time cells continue to be given off from the thickened 

 epiblast, and of those already given off many show nuclear figures 

 (fig. 8) indicating rapid division. 



7. While the ganglion is still fused with the epiblastic thickening 

 the latter begins to grow in length, and to push its way either forwards 

 or backwards, as the case may be, between the general epiblast cells 

 (figs. 40 and 41). 



8. The general epiblast cells thus pushed away are probably lost 

 (figs. 40 and 41, i.e.). 



9. Concomitantly with this growth of the sensory thickening, the 

 ganglion begins to separate from the skin, and so comes to lie deeper 

 in the mesoblast (fig. 35). As it separates there arises a nerve from 

 the sensory thickening (figs. 11, 13, &c). This nerve grows centri- 

 fugally from the ganglion, arising from the elements of the thickening, 

 and being in fact split off from the latter along its whole length. It 



1 Beard, "On the Cranial Ganglia and Segmental Sense Organs," 'Zool. Anzeig.,' 192, 

 1885; also, on some points, Spencer, "Notes on the Development of Rana temporaria," 

 ' Quart. Journ, Micr. Sc.,' Supplement, July 1885. 



