DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 525 
7. The embryo of Pl. 38, figs. 31, 82, 38. 
The embryo represented in figs. 31, 32, 33, is 19 mm. long. 
The lines of the lateral line system are complete, and with this 
stage the present study closes. The number of sense-organs 
gradually increases as the embryo grows, but the main lines 
are not different in the oldest embryos I have—at eight months, 
or 40 mm.,—and from the superficial examination of the adult, 
made near the collecting ground, I believe the lines to be 
those of the full-grown Amphibian. 
When the skin of larger embryos is examined, it is found 
that the sense-organs open to the surface by a slit-like scissure 
in the outer layer of the ectoderm. The long axis of the 
opening lies in different planes on the several sensory lines. 
Thus, on the main lateral line of the trunk the slit is horizontal, 
on the dorsal line of the trunk vertical, and on the ventral 
line the opening is often round, or may be elongated in any 
direction. In the groups of organs at the end of the snout 
the direction of the elongation also varies. In the supra- 
orbital, infra-orbital, and mandibular lines it is usually parallel 
to the direction of these sensory lines. In the ventral or 
anterior part of the double hyomandibular line the long axes 
of the openings in the outer row of sense-organs are at right 
angles to those of the inner row. These differences in the 
direction of the long axes of the openings to the sense-organs 
seem to be in no way related to their development, and are 
possibly co-ordinated with the direction in which currents of 
water flow as the animal moves. At the stage with which this 
study closes slit-like openings to the sense-organs are not 
found. 
The histology of the sense-organ resembles that of the lateral 
line organs in fishes. At the present stage of development, 
however, the difference between supporting and sensory cells 
is not sharply marked, although the external cells of the organ 
are in general somewhat flattened against the internal pear- 
shaped cells. 
vou. 38, PART 4,—NEW SERIES. NN 
