528 A. B. DAWSON 
B. Evidence of the presence of two types of glands 
1. Interature. The literature dealing with the skin glands, 
specifically or incidentally, is extensive, and good _ historical 
summaries have been presented by Gaupp (’04) and Nirenstein 
(08). Nevertheless, a brief statement of the views held by the 
various investigators will be given at this point. That the glands 
are of two kinds is an opinion shared by the greater number of 
investigators, but the criteria for such a distinction are not always 
the same. Early classifications into clear versus granular, 
mucous versus poison, and non-contractile versus contractile 
have been based on differences in structure and in the secretion 
produced or on physiological activity. 
The terms ‘Schleimdriisen’ and ‘Ko6rnerdriisen’ were first used 
by Englemann (’72), and later writers who recognized the exis- 
tence of two types of glands have for the most part adopted this 
terminology. Physiological studies and experiments prove clearly 
that the skin of many Amphibians secretes a poisonous substance 
(Phisalix, ’89, ’90, ’97, ’00a; Boulenger, 792; Hubbard, ’03; 
Nirenstein, ’08; Abel and Macht, ’11; Shipley and Wislocki, 
15). Since the poison was generally isolated from the granular 
glands, these were often referred to as poison glands (‘Giftdriisen,’ 
‘olandes & venin’). Other researches (Phisalix et Dehaut, ’08; 
Phisalix, 708) have shown that the mucous as well as the granular 
secretion may possess poisonous properties. Ancel (02), in 
his account of the development of the glands in terrestrial 
salamanders, uses the terms contractile and non-contractile to 
designate the two kinds of glands. This classification, however, 
has not come into common use, as many writers have described 
a muscular layer around both types. .The designation as mucous 
and granular glands, suggested by Engelmann (’72), appears to 
be open to the least objection, since practically all observers 
agree in their descriptions of the appearance of the secretion, 
while there is diversity of opinion regarding the poisonous char- 
acter of the secretion and regarding contractility. Bruno (’04), 
in Rana esculenta, classifies the glands as holocrine and merocrine. 
A few investigators have advanced evidence or expressed 
belief in one kind of gland. Bugnion (’73) found only one kind 
