DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 517 
the universality of the law of polarity, as Mall states it, is 
destroyed. 
6. The Embryo of Pl. 38, fig. 21. 
Fig. 21 shows the nerves and lateral line system of an 
embryo in which pigment has begun to appear in the skin, and 
when the stem of the external gills has become distinctly 
visibie. The length of the embryo is 15 mm., but the varia- 
tion in length at the same stage of development is frequently 
as much as two millimetres. 
On the supra- and infra-orbital lines sense-organs can now 
be distinguished in the skin, when removed and studied by 
transmitted light, although sections still show a continuous 
deep ridge, as in fig. 22, which passes through the infra-orbital 
line. The supra-orbital line has extended ventrally, and now 
passes around the nasal epithelium. The dorsal part of the 
line has given rise to a double row of sense-organs, as has also 
the median part of the infra-orbital line, where this line crosses 
the space between the eye and nose. The number of sense- 
organs is not constant, and gradually increases as the embryo 
grows older. Moreover where the lines are double a continuous 
area of deep sensory ectoderm connects the two rows of organs, 
in which further sense-organs may develop later between the 
rows now found. 
On the hyomandibular lines distinct sensory spots have not 
yet appeared. The dorsal part of the primitive ridge has 
become double, and has also extended in an anterior direction, 
as indicated by the two small branches that run forward from 
the hyomandibular nerve. The mandibular ridge now meets 
the anterior extremity of the ventral longitudinal ridge near 
the median plane of the embryo, and posterior to the mouth. 
At the corner of the mouth the mandibular line passes from a 
direction nearly horizontal to one nearly vertical. The line, how- 
ever, curves on to the ventral surface of the embryo, and thus 
approaches again the horizontal plane, although in a direction 
at right angles to that of the dorsal part of the line. The 
posterior or hyoid part of the hyomandibular line is little 
