BRANCHIAL SENSE ORGANS IN IOCHTHYOPSIDA. 101 
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 | 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 situate the 
rudiment of the primitive branchial sense organ of 
that root (figs. 12 and 18). 
6. For some time cells continue to be given of 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 centrifugally from the ganglion, arising from 
the elements of the thickening, and being in fact split off from 
the latter along its whole length. It is the so-called dorsal 
branch, and, as previously stated, will be here called the 
supra-branchial branch. 
10. The sensory thickening of a segment, which gives rise to 
' 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. Sci.’, Supplement, July, 1885. 
