26 THYMUS AND PARATHYROID GLANDS 



Similarly it was found in Ambystoma opaciim as well as in Amby stoma 

 maculatum that the acute tetanic attacks disappear entirely after a 

 certain stage is reached, though the animals continued to be fed ex- 

 clusively on thymus gland. When the larvae reach a stage in which 

 they resemble normal larva? ready for metamorphosis, it becomes more 

 and more difficult to induce an acute tetanic attack and no animal 

 ever showed such attacks after it had metamorphosed, in spite of 

 thymus feeding. 



Thymus Gland Contains a Tetany-Producing Substance. 



The question arises: Why does the thymus gland when fed to 

 salamander larvae cause tetany only during a certain developmental 

 period, and why is it ineffective before and after this period? In 

 order to give an answer to this question it was necessary to make a 

 histological examination of the larvae. 



As regards the beginning of tetany several larvae which had been fed 

 thymus were examined histologically just before and after the begin- 

 ning of tetany. In a number of Ambystoma opacum larvse of the same 

 age and hatched from eggs of the same female {Ambystoma opacum, 

 1917) tetany was observed in a few individuals as early as 4 weeks 

 after hatching; in these few individuals, however, it did not reappear 

 until 6 to 7 weeks after hatching, and at this time the majority 

 of the larvae developed tetany. At this time larvae of this set with 

 and without tetany were killed and preserved for examination to 

 find in which point they differed. A preliminary microscopical study 

 of the anatomy and histology of one tetanic and two pretetanic 

 larvie demonstrated a noteworthy dift'erence in regard to the devel- 

 opmental state of the thymus glands, but in no other respect. 



It was found that the larvae which had not yet developed tetany 

 had smaller thymus glands than the larva which had already fallen a 

 victim to tetany. The thymus glands of the latter larva are more 

 than twice as large as those of the pretetanic larvae, though the differ- 

 ence in length of the entire animals is only 33 per cent, the tetanic 

 specimens being the larger ones. This is shown in Fig. 1 in which 

 the three left thymus glands of a pretetanic larva (left vertical row) 



