INTEGUMENT OF NECTURUS MACULOSUS 547 
neither duct nor a muscular layer (fig. 9). The muscle cells 
grow down from the intercalary cells and become differentiated 
into fibers about the outer surface of the gland sac. The duct 
is very simply formed and is equivalent to a large intercellular 
space. Apparently it does not always extend entirely to the 
surface of the epidermis, since in many cases the expulsion of 
the secretion results in an upheaval of the upper epidermal layers 
in the region immediately above the upper pole of the gland. 
According to Fano (’03), the granular glands in young Ambly- 
stoma function without a duct and the secretion is forced through 
the intercellular spaces of the epidermis to the outside. 
The later appearance of muscles and ducts on the granular 
glands would seem to explain why, when a violent stimulus is 
applied to the surface of the animal, all the glands are not dis- 
charged at the same time. A well-developed muscular coat is 
always present on emptied glands. If muscles were developed 
early on all glands, all the secretion would probably be expelled, 
upon the reception of the proper stimulus, and the animal left 
without adequate protection. 
d. Differentiation of the mucous gland. In very early stages 
of their development, as has been shown, it is not possible to 
distinguish between granular and mucous glands. Figure 25 
shows the earliest recognizable stage of a mucous gland. <A 
distinct lumen is present and the cells are arranged so as to form 
a low epithelium. The nuclei are spherical or ellipsoidal, show 
a distinct chromatin network and occupy the centers of the cells. 
The cytoplasm in some places appears homogeneous, in others, 
finely granular, and colors slightly in either Ehrlich’s haema- 
toxylin or thionin. A large cluster of cells, similar to those 
already described in the developing granular gland, is present 
- in the region where the gland is connected with the epidermis. 
Later stages (figs. 26, 27, 28) show the gland cells undergoing 
considerable changes in form and appearance. Stricker und 
Spina (’79) distinguish three stages in the development of the 
mucous glands, ‘Ringstadien,’ ‘Mittelstadien,’ and ‘Pfropfsta- 
dien.’ In the first stage, or ‘Ringstadien,’ the epithelial cells are 
low and surround an extensive lumen; in the last, or ‘Pfropf- 
