Allen: The Thyroid Gland and Hypophysis 



415 



Hoskins. The results are 

 most interesting. Develop- 

 ment proceeds quite normally 

 for two months or more, up to 

 the time of appearance of the 

 hind limbs. In every regard, 

 both of size-growth and differ- 

 entiation, the thyroidless 

 specimens resemble the nor- 

 mal controls up to thisperiod. 

 In normal tadpoles there is 

 before this time no accumu- 

 lation of colloid secretion in 

 the thyroid gland. This 

 period — the beginning of 

 metamorphosis — is the stage 

 at which the thyroid glands 

 begin to differentiate and to 

 actively function. In nor- 

 mal development metamor- 

 phosis follows as a direct 

 result of this development 

 and functional activity of the 

 thyroid gland, while in the 

 specimens from which this 

 gland has been removed 

 there is an almost complete 

 cessation of metamorphosis. The 

 tadpoles grow to gigantic size and they 

 have been reared to an age of three 

 years but the hind limbs remain very 

 small and in no case of complete thy- 

 roid removal do the fore limbs break 

 through the skin. It is a significant 

 fact that the tadpoles of the common 

 toad Bufo lentiginosus reach a more 

 advanced stage of development when 

 deprived of the thyroid gland than do 

 the tadpoles of the leopard frog Rana 

 pipiens. It seems clear that various 

 other factors carry development up to 

 a certain point beyond which it can 

 not proceed under normal conditions 

 without the functioning of the thyroid 

 gland. The degree to which thyroid- 

 less tadpoles may develop differs in 

 different amphibians. 



A careful study of these tadpoles thus 

 retarded in development shows that, so 

 far as we can see, the retardation affects 

 not only the superficial features of 

 limb-growth and tail-shrinkage, but 

 that it also applies to all of the internal 

 structures as pro-nephros, pancreas, 

 liver, alimentary tract, thymus gland 

 and even the process of ossification. 



BUFO (TOAD) TADPOLES AND RECENTLY META- 

 MORPHOSED TOAD. NATURAL SIZE 



(a) A thyroidless tadpole six months after normal time 

 of metamorphosis, (b) A pituitaryless tadpole six months 

 after normal time of metamorphosis, (c) A normal tad- 

 pole in limb development of corresponding degree to that 

 in a and b. (d) A normal toad immediately after meta- 

 morphosis. (Fig. 16.) 



The brain not only appears to retain 

 its larval condition when viewed ex- 

 ternally; but microscopic sections dem- 

 onstrate this fact most clearly. This is 

 especially suggestive in view of the well 

 known fact that cretins are so often 

 sub-normal mentally. All of these 

 features seem chained in their embry- 

 onic conditions. In striking contrast 

 to these somatic features we find that 

 the germ glands and germ cells continue 

 in their development. Not only do 

 they grow in size, but they continue to 

 differentiate, ripe sperm developing in 

 the testis of Rana pipiens larvae pre- 

 vented from metamorphosing by thy- 

 roid removal. This is evident some 

 four or five months after the time of 

 normal metamorphosis. In the normal 

 controls spermatogenesis does not oc- 

 cur until about this length of time after 

 metamorphosis. While oocytes have 

 not become mature in these thyroidless 

 tadpoles, they have at least undergone 

 increase in size far beyond that reached 

 at the time of normal metamorphosis. 

 It thus seems clear that there is a 

 radical difference between the somatic 

 structures and the germ glands. The 



