ORIGIN—TASTE-BUDS OF ELASMOBRANCHS 49 
in this form to distinguish ectoderm and entoderm by differences 
in staining properties and the presence of large yolk granules 
in the endodermal cells. After this rupture the limits of the 
two are more difficult to determine. Evidence, however, of 
the degeneration of the endodermal lining of the pharynx or 
of active extension of the ectoderm is wholly lacking. The 
hypophysis soon loses its primary connection with the ectoderm 
from which it arises and from it therefore evidence of ectodermal 
ingrowth into the mouth is not obtainable as in Amniote embryos. 
As the embryo continues to grow the upper lip region is pushed 
backward slightly and the ectoderm is carried with it forming 
the dental ridge, as is shown in figure 16. 
Throughout the formation of the pharynx and of pharyngeal 
structures, such as the thyroid and gill pouches, it is the endo- 
derm which is the active layer. And it is within the endoderm 
that the taste-buds first make their appearance in a 45-mm. 
embryo by the local thickening of the epidermis and the differ- 
entiation of cells of the stratum germinativum. To assert that 
the taste-buds in the pharynx of Squalus are ectodermal would 
necessitate the assumption—for which there is not a particle 
of direct evidence—that the primary endodermal lining of the 
pharynx is completely supplanted by ectodermal ingrowth. 
In connection with the study of the histogenesis of taste-buds 
in Squalus another problem presents itself in the appearance in 
late embryonic stages of pharyngeal placoid scales, the distri- 
bution of which, as is shown in figure 29, is somewhat more 
restricted than that of the taste-buds. There are relatively 
few scales upon the roof of the pharynx, while they are abundant 
upon its floor, where in the region of the basibranchial cartilages 
they tend to conceal the taste-buds. The number of scales in 
the pharynx varies considerably in different individuals. In one 
individual examined there were so few that they could be detected 
only by boiling the epithelium of the roof of the pharynx in 
strong KOH solution. Yet in some individuals they may be 
easily distinguished with a hand lens or by rubbing the fingers 
anteriorly over the surface of the pharynx, since the scales like 
the pharyngeal teeth of teleosts are directed backward. 
