DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 515 
The growth of the lateral line of the trunk is chiefly through 
division and migration of cells that originally formed the 
anterior extremity of the line, and were in contact with the 
vagus ganglion. The cells that compose the growing lateral 
line are much more free from yolk than are the cells of the 
skin through which the line ploughs its course. The sensory 
ridge is thus sharply distinguished from neighbouring tissue. I 
believe, however, that the ridge is not exclusively composed of 
cells foreign to the segments through which it passes, but that 
a few cells at each side of the ridge join those that have 
advanced from anterior segments. 
Pl. 36, fig. 19, from an embryo at the stage of fig. 1, on 
which the plane of the section is indicated, shows the relative 
amount of yolk in the cells of the lateral line ridge and in 
those of the adjacent skin. Fig. 20 is a cross-section through 
the skin in a plane given on fig. 18, and shows the present ap- 
pearance of the skin where in the earlier stage (fig. 1) a deep 
lateral ridge was found. I would call attention to two facts: 
first, that the lateral nerve (fig. 20, /. n.), consisting in cross-sec- 
tion of one nucleus and a small bundle of fibres, is far too 
small to account for the disappearance of a ridge once as deep 
as that of fig. 19; secondly, the even distribution of yolk 
granules in fig. 20 shows that the deeper cells of the ridge, 
which were free from granules, have not remained in the skin 
after ceasing to form a sensory ridge. 
What, then, became of these cells? The answer is given in 
fig. 5, which illustrates one of the most peculiar phenomena in 
vertebrate development with which I am acquainted. Those 
lateral line cells that find themselves in a position with which 
for some reason they are dissatisfied, leave that position, and 
making their way over the heads of their neighbours, between 
the outer and inner layers of the skin, crowd themselves down 
into a front place in the advancing line, with a self-seeking in- 
dependence that is almost human. ~ 
In describing the development of the sea-bass, Wilson 
(39, p. 239) also calls attention to the strangely independent 
action of individual cells, which are evidently under no common 
