556 A. B. DAWSON 
It has already been stated that Weiss (’08, ’15) regards the mixed 
condition in frog tadpoles as arising from the metamorphosis 
of the mucous epithelium into a poison epithelium, the process 
beginning in the cells near the neck of the gland. Although the 
conditions found in Necturus resemble very closely those 
described by Weiss, still it seems impossible to adopt his inter- 
pretation of the origin of mixed glands. All the evidence obtained 
from a study of the glands of adult Necturus indicates strongly 
that we are not dealing with a metamorphosis of mucous cells 
into granular cells, but that we have granular cells being differ- 
entiated from the interealary region, moving down all around the 
wall of the gland and gradually replacing the mucous epithelium. 
All the glands with a mixed epithelium possess a fully developed 
duct. The mucous cells which are being displaced are completely 
transformed into secretion, appearing as they do in mature func- 
tioning glands, with their walls entirely gone and their nuclei flat- 
tened and densely stained (fig. 37). On the other hand, the gran- 
ular cells, especially those near the duct, usually appear young. 
The granular secretion is in the early stage of its elaboration and 
appears in all fixations as large discrete granules. The walls 
of the granular cells are distinct. Furthermore, on the side next 
the mucous secretion the granular cells are generally somewhat 
flattened and on this flattened surface lie the densely stained 
nuclei of the old mucous epithelium which is being displaced. 
In figure 33 a later stage of displacement than that represented 
in figure 37 is shown. On the left side of the gland, immediately 
below the intercalary region, are two undifferentiated granular - 
cells. They are clear and resistant to stain. Within their 
cytoplasm the fine wavy lines observed in the cells of developing 
granular glands can be distinguished. Below them is a large 
granular cell whose wall has disintegrated and allowed the gran- 
ules to escape into the mucous secretion. On the right side of 
the gland is an immense granular cell, greatly distended with 
secretion, but with its cell wall still intact. 
The conditions just described appear to furnish no evidence 
in support of the theory of the origin of granular cells from 
differentiated mucous cells. In all mixed glands found, the 
