548 A. B. DAWSON 
stadium’ the cells are very tall and practically fill the entire 
lumen. Between these two extremes lies the ‘Mittelstadium.’ 
In Necturus similar conditions are found and the various forms 
of the epithelial cells in developing glands appear to be due 
to the elaboration of the secretion. Figure 26 shows a gland 
in which the formation of secretion is just begun. The cells 
are cubical, with central nuclei, and are arranged in a single 
layer about a wide lumen. Just external to the definitely 
arranged epithelium, flattened cells (cl’) occasionally are found. 
They are apparently of epithelial origin and probably represent 
cells which are migrating downward from the epidermis, later 
to take up positions in the mucous epithelium. As more secretion 
is formed the gland cells enlarge greatly and extend further into 
the lumen (fig. 27). The nuclei, which formerly lay near the 
center of the gland cells, are now found close to the gland wall, 
being forced outward and greatly distorted by the pressure of 
the secreted substance. In figure 28 a condition corresponding 
to the ‘Pfropfstadium’ is shown. The. lumen of the gland has 
completely disappeared. The cells are enormously enlarged and 
elongated, and their entire protoplasm seems in most places to 
be replaced by mucus. In the upper part of the gland are several 
cells which have ruptured at their outer ends and their slimy 
contents have streamed outward through the duct. In the 
mature glands, as already noted, the cell walls are usually gone, 
leaving the secretion free within the body of the gland. In a 
few cases, however, only the ends of the cells have disappeared 
and on the sides the cells are still definitely limited. 
In mucous cells the process of secretion is commonly described 
as beginning with granule formation, the mucigen granules later 
becoming changed into clear vesicles of mucus. Many writers 
have described a distinct granular stage in the elaboration of 
the secretion in the mucous glands of the amphibian skin (Nico- 
glu, 93; Nordenskidéld, ’05; Arnold, ’05; Nirenstein ’08). 
In Necturus the mucous secretion makes its first appearance 
in the cytoplasm near the free ends of the cells, but no distinct 
granular stage in its formation was observed. Many young 
cells, however, do appear finely granular, but this condition 
