516 BENNET M. ALLEN 



journal. While he shows that the development of the gonads 

 and germ cells is not accelerated by thyroiel feeding, this paper 

 shows that their development is in no wise affected by the ab- 

 sence of the thyroid gland. Swing e s 1 o ' s that the develop- 

 ment of the gonads anel germ cells is very sensitive to food condi- 

 tions. In this way, we might explain the incomplete sexual de- 

 velopment of cretins. It woulel seem highly probable that here 

 the thyroid glands are merely the indirect cause, in that they 

 disturb the general bodily growth anel metabolism to such an 

 extent as to produce a condition of under nourishment. 



It is interesting to speculate upon the bearing of these facts 

 upon Weissmann's hypothesis of the independence of the germ 

 plasm from the soma. Malnutrition naturally affects both, since 

 it regulates the materials that go into the formation of both 

 germ cells and soma. Here with the thyroiel glanel however, 

 we are dealing with a factor that has to elo with the entire proc- 

 ess of metamorphosis beyond the stage of early limb develop- 

 ment. Just as the thyroid gland is not essential to the earlier 

 phases of embryonic development, so is it likewise not at any 

 time essential to the development of the germ cells. One should 

 not expect the growth of the germ cells to be governed by the 

 thyroid secretion, when the earlier developmental stages of the 

 embryos that arise from them are shown to proceed normally in 

 the absence of the thyroid gland. The whole question of the 

 factors regulating the development of the somatic portions of the 

 gonads is another matter. In Rana pipiens, the general features 

 of structure of the gonads are laid down before the period when 

 the soma ceases to differentiate. We know all too little regard- 

 ing the interesting question of the influence of the germ cells 

 upon the development of the gonads, and it would at present be 

 difficult to say whether the somatic portions of the latter are 

 governed more by the germ cells within them or by the factors 

 that regulate the development of the soma in general. 



It will be extremely interesting to analyze further the effects 

 of the thyroid secretion upon the differentiation of organs, upon 

 histogenesis, upon the problem of senescence, and upon regenera- 

 tion. The present paper shows a most promising method of 

 attack upon the fundamental problems of development. 



