50 MARGARET H. COOK AND H. V. NEAL 
Pharyngeal scales in Squalus are similar to, although not identi- 
cal in structure with, the placoid scales of the outer skin. They 
differ from them, however, in their origin. The placoid scales 
of the outer skin and the teeth of the upper and lower jaws 
derive their enamel layer from the embryonic ectoderm. Pha- 
ryngeal scales, however, form their enamel layer from the endo- 
derm. The reasons for this conclusion are identical with those 
which convince us of the endodermal origin of the taste-buds. 
The pharyngeal scales, however, arise later than the taste-buds. 
No developed pharyngeal scales appear in the pharynx of a 
‘pup’ stage in which the taste-buds are well developed and numer- 
ous (fig. 28). Sections of such a stage, however, show the anlagen 
of the pharyngeal scales (figs. 4 and 9). In such a section as 
that shown in figure 4 it may be seen that the same layer of the 
epidermis gives rise to the sense-cells of the taste-bud and to the 
enamel layer of the pharyngeal scale. Entoderm, therefore, it 
would appear, may give rise not only to sense organs, but to 
scales such as usually are conceived as ectodermal, that is to say 
cutaneous, In origin. 
Against such a conclusion two chief objections may be urged: 
first, that it is highly improbable, since it conflicts with the 
principle of the specificity of the germ layers, and, second, that 
the acceptance by morphologists of the annelid ancestry of 
chordates justifies the assumption of an ectoderm-lined pharynx 
in the latter comparable with the stomodaeum of annelids. It 
is very doubtful, however, that such considerations will seem to 
many morphologists to outweigh the direct evidence from onto- 
genesis presented by Johnston and Landacre and in the present 
paper. It is hardly necessary to suggest that a biological prin- 
ciple is neither a self-evident truth nor a universal law, but a 
generalization or hypothesis usually formulated before a com- 
plete knowledge of the evidence. In the phenomena of budding 
in colonial tunicates and in the regeneration processes of chordates 
may be found exceptions to the principle of the specificity of 
germ layers. That endoderm may produce taste-buds and 
placoid scales is surely not more surprising than that muscle may 
be regenerated from ectoderm or the lens of the eye from mesen- 
