52 MARGARET H. COOK AND H. V. NEAL 
3. Since all other nervous receptor cells are ectodermal in 
origin, the presumption is wholly in favor of the ectodermal 
origin of the hair-cells of the taste-buds. This deduction, how- 
ever, is based upon the assumption that the hair-cells of the 
pharyngeal taste-buds are nervous receptors. If they are glan- 
dular, as Botezat (’10) and Parker (’12) have assumed, the deduc- 
tion is a logical non-sequitur. 
4. That an ectodermal stomodaeal invagination occurs in 
vertebrate embryos is an established fact. The persistence in 
some animals of the connection of the ectodermal hypophysis 
with the roof of the embryonic mouth makes it probable that 
in these forms the ectodermal ingrowth extends as far poster- 
iorly as the eustachian tubes. A relatively slight continuation 
of this process of ingrowth would line the pharynx with ectoderm. 
In the light of this positive evidence of extensive invagination 
of the ectoderm, disagreement with the opinion of Keibel (’12, 
p. 183), that “it is difficult to suppose that the ectoblast has 
penetrated into the region of the larynx,’ might seem not 
unreasonable. 
5. The divergence in the conclusions of Kingsley aid Thyng 
(05) and Johnston (’10) regarding the amount of the ectodermal 
invagination in Amblystoma might appear to justify some doubt 
as to the certainty of the conclusion reached by the latter. 
Johnston seems not to have taken into consideration vine facts 
presented in the former paper. 
The following considerations, however, favor the wonelon 
that the pharyngeal lining with its associated taste-buds and 
placoid scales are derived from the entoderm: 
1. The similarity of structure of the pharyngeal lining and 
the skin by no means proves a similar genesis or derivation. 
The similarity may be the result of convergence. Many instances 
of the convergence of organic structures from dissimilar begin- 
nings are known. The force of the doctrine of the specificity 
of the germ layers has been greatly weakened by abundant 
evidence of the plasticity of regenerating tissue. 
2. The pharynx of all vertebrate embryos is lined primarily 
with endoderm (figs. 5 to 27 of this paper). Direct evidence 
