.46 E. R. HOSKINS AND M. M. HOSKINS 



None of the 1917 thyroidless larvae were killed in September, 

 but in October the testes of those which were examined were 

 seen to be in an advanced stage of differentiation, with some 

 fully formed spermatozoa (fig. 108). We do not know just when 

 spermatozoa are first produced normally in R. sylvatica, but 

 Allen ('18) states that this occurs in R. pipiens during the winter, 

 so it is probable that this process has been hastened in these 

 thyroidless larvae. In the second spring the testis had developed 

 still further in our thyroidless larvae, and in the following July 

 it appeared to be mature (fig. 109). It was joined to the kidney 

 by the usual efferent ducts, in which spermatozoa were found. 

 The testis becomes connected with the kidney in these larvae 

 probably during the first autumn or winter. 



In the larger 1918 thyroidless larvae killed in July spermato- 

 genesis was less advanced than in the larger but older 1917 

 larvae killed in October; hence the rapid growth of the 1918 

 thyroidless larvae did not accelerate the rate of differentiation 

 of the testes as much as that of the ovaries, although it affected 

 it to some degree. 



From the condition of the testes in the oldest thyroidless 

 larvae it might be argued that experimental neoteny, as Allen 

 ('18) has termed it, has been produced. The term neoteny, how- 

 ever, has always implied the ability to reproduce, and objection 

 may be raised to its use here since the females did not reach 

 complete sexual maturity and reproduction by thyroidless larvae 

 has not actually occurred. 



SUMMARY AND CONCLUSIONS 



The thyroid anlage may be removed from j^oung amphibian 

 embryos before it has begun to differentiate. The thyroidless 

 larvae in which muscular defects are not produced by the oper- 

 ation grow more rapidly than the controls and are often twice 

 as large in volume as the normal larvae at the time when the 

 latter reach their maximum size. Ultimately they may become 

 more than three times as large as the controls. The submaxillary 

 muscles are especially liable to injury by the operation and larvae 

 so injured grow less rapidly than the controls, since they suffer 

 from inanition, owing to difficulty in eating. 



