DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 497 
derm cells is here cut off to form the ganglion of the lateral line. 
As I have mentioned above, the long axis of the cells compos- 
ing this ganglion is at right angles to that of the cells in the 
neural part of the vagus ganglion, but in the same plane as that 
of the cells in the commissure. The consequence is that when 
the cells of the lateral line form nerve-fibres, the 
greater part of the fibres, following the path of 
least resistance, pass over the commissure and enter 
the brain with the root of the glossopharyngeal 
ganglion. Cells which lie on the inner side of the lateral 
line ganglion and in immediate contact with the primitive 
vagus ganglion send fibres to the brain by the vagus root, 
but the ninth nerve is the chief root of the lateral line 
ganglion. 
Pl. 36, fig. 5, is from a horizontal section through the 
dorsal and median lateral lines of the trunk at the time of their 
beginning. The plane of the section is shown in fig. 1. One 
who has seen the growing lateral line plough its way through 
the skin, leaving a row of sense-organs in the wake, would 
perhaps imagine the structure at the right of the section to be 
a large sense-organ which the advancing line, cut longitudi- 
nally at the left of the section, had formed in its path. Such, 
however, is not the case. The section drawn is from the series 
reconstructed in Pl. 38, fig. 1, and the position of the lateral 
ridge in relation to the somites has been carefully determined. 
There is no sense-organ at a later stage in the place now occu- 
pied by the group of cells in question. The first sense-organ 
of the dorsal lateral line does not lie anterior to the third 
intersegment back of the ear. The group of cells now found at 
the second intersegment consequently either push their way 
farther, or leave the skin in giving rise to ganglion and nerve. 
Moreover the fate of that part of the main line which has as 
yet developed is not different, and the cells of this deep ridge 
also either advance in the skin, or give rise to ganglion or 
nerve, for the first sense-organ of the main lateral line does 
not fall anterior to the fourth intersegment. The entire 
ectodermic thickening represented in fig. 5 ultimately dis- 
