INTEGUMENT OF NECTURUS MACULOSUS 557 
mucous epithelium was completely metamorphosed into secre- 
tion and the cell walls were gone. It is difficult to think that 
poison cells, possessing definite walls and capable of producing 
immense quantities of secretion could arise from these completely 
differentiated and apparently almost exhausted mucous cells. 
It appears much more probable that in a mucous gland the inter- 
calary region, itself of epidermal origin, could furnish cells which 
might develp under suitable conditions into granular cells, 
since the germinative layer of the normal epidermis produces 
gland buds which may become either granular or mucous glands. 
The displacement of the mucous nuclei from the gland wall and 
their subsequent position on the lumen-surface of the granular 
cells seems to furnish almost conclusive evidence that the granu- 
lar cells are not developing from mucous cells, but are displacing 
them. 
In one instance a peculiar mixed gland was found (fig. 36). 
Its appearance suggests that the mixed condition may in some 
cases become permanent. The gland possessed a well-formed 
duct and intercalary region. One half of the alveolus was 
occupied by the typical mucous epithelium and secretion. On 
the other half, a definite muscular layer apparently in the con- 
tracted state was present and, resting upon it, was a single layer 
of tall columnar cells which possessed all the characteristics of 
potential granular cells. The only possible interpretation of this 
structure seems to be that it is a permanent mixed gland with 
the granular and mucous epithelia existing side by side. Around 
the granular portion a muscular layer has developed, but, as 
would be expected, none has appeared around the mucous half 
of the gland. The presence of the clear epithelium resting upon 
a contracted muscular layer is difficult to explain. But in the | 
light of conditions found in regenerating granular glands, to be 
described later, the most plausible explanation would be to 
regard the clear, columnar cells as a new epithelium which is 
replacing exhausted granular cells whose contents have been ex- 
pelled by the contraction of the muscular wall. 
