Regeneration 155 



accepted this view and the writer made use of it in his 

 first experiments on regeneration and heteromorphosis 

 in animals. ' At that time the idea of the existence of 

 such specific organ-forming substances was received 

 with some scepticism, but since then so many proofs 

 for their existence have been obtained that the idea 

 is no longer questioned. Such substances are known 

 now under the name of "internal secretions" or "hor- 

 mones"; their connection with the theory of Sachs was 

 forgotten with the introduction of the new nomenclature. 

 It may be well to enumerate some of the cases in 

 which the influence of specific substances circulating 

 in the blood upon phenomena of growth has been 

 proven. One of the most striking observations in this 

 direction is the one made by Gudernatsch on the growth 

 of the legs of tadpoles of frogs and toads. ^ The young 

 tadpoles have no legs, but the mesenchyme cells from 

 which the legs are to grow out later are present at an 

 early stage. From four months to a year or more may 

 elapse before the legs begin to grow. Gudernatsch 

 found that legs can be induced to grow in tadpoles at 

 any time, even in very young specimens, by feeding 

 them with the thyroid gland (no matter from what 



' Loeb, J., Untersuchungen zur physlologlschen Morphologie der 

 Tiere. I. Heteromorphose. Wiirzburg, 1891. II. Organbildung und 

 Wachsthum. 1892. Reprinted in Studies in General Physiology. 

 Chicago, 1906. 



* Gudernatsch, J. P., Zentralhl. /. Physiol., 1912, xxvi., 323; Arch, f, 

 Entwcklngsmech., 1912, xxxv., 457; Am. Jour. Anat., 19 14, xv., 431. 



