Metamorphosis of Axolotls 
57 
as to the cause of the neotenia characteristic of these amphibians; 
The work of previous authors referring to glands with internal 
secretion gives no information on the topography of the thyroid 
gland of Amblystoma; even Bo lau does not allude to the question 
in his paper: „Glandula Thyreoidea und Glandula Thymus der 
Amphibien“, although he studied Amblystoma and described its 
thymus. I therefore looked for the thyreoidea above the aortic arches, 
according to their location in newts, frogs and salamanders. These 
investigations gave at first no result so that, in my preliminary 
note, I supposed that the thyroid gland of axolotls is very feebly 
developed or totally absent. Guided by what I observed on 
microscopical dissections of salamander larvae, I looked for the 
thyreoidea of the axolotl in the region between the arches of the 
lower jaw and, by means of a binocular magnifying glass, I suc¬ 
ceeded in finding that gland. It may be found in the following 
manner: the animal is laid on its back and the skin is dissected 
along the medial line of the throat; then we remove the mylohyoideus 
muscle. In larvae as well as in specimens undergoing metamor¬ 
phosis, and in those already transformed, the thyroid is situated 
on both sides on the sternohyoideus close to the caudal part of the 
geniohyoideus muscle, somewhat covered by it (Fig. 11). Several 
times I observed a third gland, besides these two- symmetrically 
placed bodies. It was 6 mm. long, narrow, situated on the middle 
line between the two geniohyoidei muscles, somewhat above the 
coupled thyreoidea. The thyroid gland is difficult to find, not 
only on account of its small size (in animals of 20 cm. length, 
the length of the thyreoidea attains 3 to 4 mm., its breadth 1 to 
1-5 mm.), but especially because it is of pale glassy appearance, 
so that it can scarcely be discerned from the surrounding muscles. 
Even macroscopically, some differences in the structure of the 
thyroid gland of normally fed larvae and of specimens fed on 
thyroidine may be noticed. The thyreoidea of larvae lies far from 
the blood-vessels; its characteristic vesicular structure is hardly 
discernible with the naked eye or by means of a binocular magni- 
fying glass. In specimens undergoing metamorphosis under the 
influence of thyroidine-food, or in transformed animals, the thyroid 
gland adheres closely to the blood-vessels; its single vesicles are 
observable by the naked eye. 
Phot. 3 represents a microscopical section of the thyroid'gland 
