194 



THE AMERICAN NATURALIST 



[Vol. LV 



growth and development, exhibit a remarkable resem- 

 blance, and the secretions of these two glands can replace 

 each other to some degree, but for the most part are 

 specific. 



. Among those who have worked out these facts, J. F. 

 Gudematsch, Leo Adler, Bennet Allen and his pupil 

 W. W. Swingle, E. E. Hoskins and M. ^1. Iloskins, and 

 P. E. Smith deserve the greatest credit. Since much of 

 their success is due to the extirpation of the endocrine 

 glands in early embryonic stages, we should mention here 

 also the names of three investigators, namel^y Gustav 

 Born, Herman Braus and Ross Gr. Harrison, who have 

 elaborated the delicate technique employed in the extir- 

 pation experiments and thus have made possible the 

 progress which has been derived from them. 



We will begin with the thyroid mechanism, as it has 

 been studied more thoroughly than other glands, and in 

 fact seems to be the chief factor in the control of growth 

 and development. Its study in the amphibians as well 

 as the entire work on amphibians w^as initiated by the 

 well-known experiments on thyroid-feeding to tadpoles 

 as carried out by Gudematsch (1). 



If tadpoles are fed fresh thyroid gland or are kept in 

 water to which minute amounts of thyroid extract are 

 added, a remarkable acceleration of development takes 

 place. This development is the more conspicuous as it 

 may occur with complete absence of growth. In tadpoles 

 it is characterized especially by the sudden development 

 of the fore limbs, by the atrophy of the tail, a sudden 

 protrusion of tlie exc-balls {■2), by the rapid shortening 

 of the s])iral uut, by the pivcocions atrophy of the organs 

 of th.. Ini'vnl iiKnitli, which are replaced by the rn-u' mouth 

 (:V), and l.y pivcceimis o-silicat ion (4). Thrsc experi- 

 ments have been rei)eate(l with the larv;v of salamanders, 

 in which the precocious occurrences of the first moult, the 

 atrophy of the gills and absorption of the tin of the tail, 

 are most conspicuous effects of the thyroid applica- 

 tion (5). 



