IX THE SKIN 183 



of opinion regarding many of the most important features 

 of their structure and functions. Two varieties of gland are 

 commonly distinguished which may be designated as the 

 mucus glands and the poison glands. While Heidenhain, 

 Nicoglu, and others regard these two types of gland as 

 specifically distinct, other investigators (Calmels, Leydig, 

 Sczcesny, Junius) consider them as different phases in the 

 development of one and the same gland. However this 

 may be, the glands of the frog's skin may be grouped into 

 two classes which are structurally and functionally different, 

 and we shall describe them separately without regard to the 

 question g.s to whether they are genetically connected. 



The mucus glands are smaller and much more abundant 

 than the poison glands, and are found over practically the 

 entire surface of the body. In some places they are so thick 

 that they nearly touch. In Rana fusca, according to Engel- 

 mann, they average about sixty to each square millimeter of 

 surface. Their ducts are narrow, and lined with a layer of 

 small flattened epithelial cells. The body of the gland is 

 lined with epithelial cells which form a single layer except 

 near the opening of the neck, where there are two layers. 

 It is this epithelium which forms the mucus which is dis- 

 charged into the lumen of the gland, and poured out through 

 the neck over the surface of the skin. The appearance of 

 the secreting epithelium varies greatly in different glands. 

 In some cases, more often in the smaller glands, the epi- 

 thehal cells are low, clearly marked off from each other, and 

 from the large lumen of the gland, and contain nuclei which 

 take up a large part of the cell. In other glands the cells 

 are elongated so that they fill a large part of the lumen, the 

 nucleus is relatively small, and situated near the base of the 

 cell, and numerous granules occur toward the free ends. 

 During secretion these granules swell up, and become con- 



