INTEGUMENT OF NECTURUS MACULOSUS 555 
According to Nirenstein, the double structures arise in the course 
of the development of poison glands from mucous glands, and 
are not the result of any process of replacement of an exhausted 
epithelium by a new one. 
Furlotti (’09,’11) also describes mixed glands in Triton cristatus. 
According to him, the gland epithelia are of two distinct types, 
and the mixed condition may be permanent. The granular 
portion of the gland is sometimes in communication with the 
duct and at other times the mucous secretion is discharged 
through the outlet. Weiss (’08, 715) states that in the frog the 
granular glands are directly differentiated from those of the 
mucous type and that in this process of differentiation mixed 
glands are temporarily formed. ‘These double structures, how- 
ever, are quite unlike those described in Salamandra, Triton, 
and Plethodon, since the mucous cells never appear as small sacs 
within the large granular glands. ‘The mucous cells are found in 
the fundus of the gland and the granular cells lie near the base 
of the duct. This condition, according to Weiss, is due to the 
fact that the process of metamorphosis of mucous cells imto 
granular cells begins at the duct, and the rapidity with which 
the metamorphosis is accomplished explains why mixed glands 
are so rarely found in preparations of the skin. 
2. In Necturus. The mixed glands found in Necturus resemble 
those described by Nordenskidld (05) and Weiss (08). The 
granular cells occur in the upper part of the gland, just below 
the intercalary cells, while the mucous epithelium usually occupies 
the deeper part of the gland alveolus (figs. 33, 37). All stages of 
the mixed condition are found, from one in which only a small 
quantity of granular secretion is present in the region of the 
gland neck to one in which the mucous epithelium and secretion 
have been almost completely displaced and the lumen is occupied 
chiefly by the granular secretion. Im several cases fully devel- 
oped granular glands were found with plugs of mucous secretion 
in their ducts. This probably represents the last stage of dis- 
placement of the mucous secretion by the granular secretion, 
and has been so interpreted by Weiss (’08). Calmels (’83) also 
speaks of a ‘slime plug’ in the ducts of some poison glands. 
