August 31, 1917] 



SCIENCE 



217 



properly fed. The continued development of 

 their gonads and germ cells, and the normal 

 metamorphosis of the similarly fed controls 

 all show conclusively that we are here dealing 

 with conditions resulting from the removal of 

 the thyroid glands. 



It must be kept in mind that the thyroid 

 anlagen were removed at their very inception. 

 It is fair to say that in these tadpoles there 

 has never at any time been any thyroid secre- 

 tion. A careful study of serial sections has 

 demonstrated with certainty the total absence 

 of these glands in the crucial cases used as a 

 basis for this work. 



The germ glands of the thyroidless tadpoles 

 develop quite normally throughout, both as 

 to structure and rate of development. When 

 the operated tadpoles begin to lag behind the 

 controls in general bodily differentiation, the 

 gonads have already undergone sexual differ- 

 entiation but have not yet shown any tend- 

 encies toward spermatogenesis. The remark- 

 able feature of these experiments is seen in 

 the fact that although differentiation of the 

 soma halts completely at this' early stage, the 

 gonads continue to develop normally, keeping 

 pace at every stage with the development of 

 the gonads in control specimens. This applies 

 both to the development of the gonads as a 

 whole and to the development of sperm and 

 ova. 



At the time of metamorphosis the testes of 

 both controls and thyroidless specimens 

 showed similar dimensions. In no cases were 

 there evidences of spermatogenesis. A thy- 

 roidless tadpole killed September 14 showed 

 very active stages of spermatogenesis termin- 

 ating in the production of many spermatids; 

 but as yet no spermatozoa. Ripe spermatozoa 

 were, however, found in a thyroidless tadpole 

 killed December 15. In this case they were 

 few in number, but in a thyroidless tadpole 

 killed February 7, to which reference was 

 made above, they were very numerous. This 

 latter specimen had testes nearly twice as large 

 as those of yoimg frogs at the time of meta- 

 morphosis and of course very far beyond the 

 condition found in tadpoles of a similar stage 

 of body differentiation. 



No less striking were the conditions in fe- 

 male specimens. At the time of metamor- 

 phosis, the central cavity of the ovary had 

 formed, but the organ had not yet become 

 folded as was later to be the case. All but a 

 few scattered germ cells had become converted 

 into large oocytes. An average of 12 of the 

 largest measured showed dimensions of .2025 

 mm. X -2502 mm. As time passed, the thy- 

 roidless tadpoles showed continued growth of 

 the ovaries. On February 14 they reached a 

 size twice as great as at the time of metamor- 

 phosis. During all this time the oocytes of 

 the thyroidless tadpoles steadily increased in 

 size, as seen in specimens killed from time to 

 time. In a thyroidless tadpole killed Febru- 

 ary 14 the average dimensions of the oocytes 

 were .4027 mm. X -5207 mm. It is quite inter- 

 esting to compare with this the conditions 

 found in a normal young frog that metamor- 

 phosed last summer, living in the open and 

 reaching a length of 48 mm. When it was 

 killed March 13 the larger ova were seen to 

 have reached an average size a trifle below that 

 of the case just given, namely, .4123 mm. X 

 .4540 mm., although the ovaries as a whole were 

 somewhat larger, 8.05 mm. X 99-2 mm. as com- 

 pared with 6.6 mm. X '^■^ nun- in the thyroid- 

 less tadpole killed March 15. This is prob- 

 ably due to the difference in bodily nutrition 

 and is about proportional to the length of 

 body of the two specimens compared. 



From all this evidence I feel that we are 

 justified in stating that the absence of the 

 thyroid gland does not affect the development 

 of the gonads or germ cells up to the time of 

 sexual matin-ity in the male nor does it hinder 

 the development of the ovary and ova, at least 

 up to the period when the ova are visible with 

 the naked eye. It is, of course, possible that the 

 astonishing modifications of the soma may 

 later secondarily affect the nutrition of the 

 developing ova, but this is beyond the point. 



These results are in line with some unpub- 

 lished work by Mr. Wilbur W. Swingle, who 

 at my suggestion studied the effects of thyroid 

 feeding upon the germ glands and germ coUs 

 of Eana pipiens tadpoles. He shows that 

 while this brought about the well-known 



