ORIGIN—TASTE-BUDS OF ELASMOBRANCHS Dil: 
chyma, as has been demonstrated (Morgan, ’01). In our con- 
clusions concerning organic processes we have to reckon with the 
fact of organic plasticity as well as with that of specificity of 
tissue. 
In the light of the considerable doubt that attaches to the 
annelid hypothesis of chordate ancestry, it is perhaps sufficient 
simply to suggest that such an hypothesis forms a most insecure 
foundation for deductions concerning the derivation of the lining 
of the chordate pharynx. The annelid hypothesis certainly seems 
less convincing than it did formerly. Yet it seems as if in our 
thinking we had tacitly assumed on the basis of that hypothesis 
a considerable ectodermal invagination into the pharynx. Such 
a prejudice is not supported by the ontogenetic evidence. 
A brief summary of the arguments which have been presented 
for and against the endodermal origin of taste-buds and pharyn- 
geal scales may now be given. In favor of the ectodermal 
derivation of these structures it may be urged that: 
1. The resemblance of the histological structure of the pharyn- 
geal epidermis to that of the mouth and skin—which are known 
to be of ectodermal origin—suggests a similar derivation for 
both. Furthermore, taste-buds and pharyngeal scales in Squalus 
structurally resemble cutaneous organs of known ectodermal 
origin. Such an argument, however, leads logically to the con- 
clusion that the mucous lining of the esophagus is also ectodermal 
—a conclusion contradicted by the ontogenetic evidence. Are 
we to conclude, also, that because the epidermis of the gills of 
fishes does not resemble that of the pharynx and skin it is there- 
fore not ectodermal in origin? 
2. Taste-buds occur in the skin of some fishes in regions where 
their ectodermal origin seems indisputable (Herrick). It may 
be urged that it is highly improbable that the same sort of 
structure should arise independently from different germ layers. 
This argument seems strengthened by the fact that associated 
with the taste-buds in Squalus are placoid scales structurally 
comparable with those derived from the outer skin. Is it pos- 
sible to believe that the specificity of the germ layers is so slight 
that identical structures may develop from both endoderm and 
ectoderm? 
