LATERAL SENSE ORGANS OF, SQUALUS SUCKLII 287 
summarized briefly. The lateral sense organs do not develop 
in situ from successive or segmental patches of ectoderm along 
the side of the body, but each lateral sensory column arises from a 
thickened area of ectoderm located on the side of the head; this 
invades the posterior segments of the body not as a segmental 
structure, but in the form of a continuous column of epithelial 
cells. The grouping of the sensory cells in small clusters occurs 
comparatively late inthe development of the embryo. It has 
been pointed out that these groups, when they do appear, are not 
segmental in the sense of the term as here employed. It is only 
in a degenerating or breaking down condition of the sensory 
ridges that isolated groups (pit-organs) of hair cells are found 
(e.g., dorsal series of sense organs in Squalus acanthias). These 
so called pit-organs show no relationship to the body segments 
either in their number or in their innervation. Further, their 
early development is identical with that of the lateral sense 
organs, the separated organs simply representing parts of what 
was earlier a continuous ridge of epithelium. So much for the 
developmental aspect. 
The opinion has already been expressed that the slight tendency 
towards segmentation as seen in the lateral sensory column of the 
adult is probably of no significance as an argument for the seg- 
mentation theory. This is one of the anatomical features, how- 
ever, which might be considered as pointing in that direction. 
Another one is seen in the innervation of the sensory epithelium 
by separate and successive ramuli (of the lateral nerve) which cor- 
respond in number and level with the surface tubules. The first 
condition named loses segmental significance when one remembers 
Fig. 7 Longitudinal section of lateral sensory epithelium showing the exten- 
sive branching of a single large nerve fiber. Pyridine silver. 1525, 2 off. 
Fig. 8 Longitudinal section of a group of hair cells, showing various relations 
of the terminal fibrillae. Pyridine silver. X 650, } off. Var., varicosity. 
Fig. 9 Section showing several slender fibrillae diverging from a large vari- 
cosity (Var.). Pyridine silver. X 650, 4 off. 
Fig. 10 Longitudinal section of the lateral sensory column, showing two 
groups of hair cells (Grp.), and a network of fibers arising from the subbasilar 
fiber zone (Fb.Zn.). Pyridine silver. XX 650, 4 off. B.Cl., basal cell; N.M.Fb., 
non-medullated nerve fibers. 
