30 THYMUS AND PARATHYROID GLANDS 



was never observed after metamorphosis and after the development of 

 the parathyroids. This suggestion finds support in the fact that in 

 the larvae of frogs and toads, which develop their parathyroids soon 

 after hatching, thymus feeding never produces tetanic convulsions . 



A. Second Mechanism to Counteract the Action of the Thymus in 

 Salamander Larvae. 



If the parathyroids are responsible for preventing tetany in Amhy- 

 stoma opacum and Amby stoma maculatum, the question may be asked 

 why the larvae of these salamanders are not normally victims of 

 tetany, since they possess six well developed thymus glands and no 

 parathyroids. It is necessary to assume that the larvae must possess 

 some other mechanism capable of inhibiting the action of their own 

 thymus glands but inadequate when an excess of thymus substance is 

 introduced into the larva. 



That such a mechanism is present becomes evident from the 

 behavior of the larvae of a third salamander species (Ambystoma 

 tigrinum) closely related to the two others. It is surprising that the 

 larvae of Ambystoma tigrinum never develop tetany even if they are 

 fed exclusively on thymus gland of which they eat enormous quanti- 

 ties. Ambystoma tigrinum larvae were fed from about the 14th day 

 after hatching, on calf's thymus exclusively; some of them were kept 

 at 25° C, others at 15°C. One of these larvae has not metamorphosed 

 yet, 60 weeks after hatching; it has been fed on thymus for more than 

 a year. None of these larvae ever showed any signs of tetany. 



Yet the larvae of the species Ambystoma tigrinum, like those of the 

 two other species, do not possess parathyroids. Evidently they must 

 have some other mechanism to counteract the tetanic effect of the 

 thymus; and this mechanism must be sufficient not only to prevent 

 tetany from the animal's own thymus but also tetany from an excess 

 amount of thymus when introduced with its food. 



CONCLUSIONS. 



From the facts stated in this paper it is evident that the thymus 

 gland of mammals contains a substance which is capable of producing 



