38 



Messrs. J. 8. Huxley and L. T. Hogben. 



varying from 27 to 32 days. Two of these animals were females who had just 

 started to lay eggs ; the very interesting fact was noticed that oviposition 

 ceased almost immediately the thyroid diet was started. (It would thus appear 

 that the katabolic action of thyroid will not permit of the anabolic activities 

 necessary for the production of eggs. On the other hand, the experiment 

 does not prove that egg laying cannot proceed during metamorphosis, for the 

 amount of thyroid given was certainly very much in excess of that necessary 

 to produce metamorphosis. It would be of great interest to attempt work 

 along these lines with amounts of thyroid (or iodothyrin or thyroxin) close to 

 the critical amount.) One immature animal was fed on thyroid while in deep 

 water; in spite of this, metamorphosis proceeded at the usual rate. There 

 were no failures to metamorphose (except the above-mentioned one on 

 iodothyrin diet) among all the survivors, and the two that died under the 

 treatment were metamorphosing regularly up to the time of their death. 

 Salamandra maculosa larvae were also metamorphosed by a thyroid diet. 

 Eight-day larvae came out on land in 13 to 16 days, and the same period of 

 time was taken by 30-day larvae. Axolotls in 1/10,000 KI solution did not 

 metamorphose, but on the contrary developed abnormally large gills. 



Experiments were also carried out by Jensen upon Necturus and Proteus. 

 One adult Necturus was at first fed on thyroid, later injected with iodothyrin. 

 The only changes noted were the resorption of one gill on one side, and a 

 slight change in pigmentation. The animal died after some time. Four 

 specimens of Proteus were also treated, some fed on thyroid, others injected 

 with iodothyrin. The animals were 12 to 24 cm. long. In all cases, there 

 was a distinct but slight atrophy of the tail-fin and of the gills, but the total 

 shrinkage of the fin in height was not more than 1 to 1*5 mm., and after 

 6 months it regained its original size. Thus, with animals of this size, only 

 slight effects could be obtained, indicating that in these permanently neotenic 

 forms, the tissues are not sensitive to thyroid in the same way as in normally 

 metamorphosing or facultatively neotenic species. Similar treatment failed 

 to produce metamorphic effects in the Ammocoete larva of the Lamprey. 



Jensen further notes that in many, but not all, of his treated Axolotls, 

 pathological symptoms occurred as a result of thyroid or iodothyrin. The 

 animals refuse to eat, are usually sluggish when stimulated, rush about 

 madly for several minutes, and exophthalmos of a very pronounced - type 

 appears. This exophthalmos, it is interesting to note, disappears with the 

 other pathological symptoms from a few days before metamorphosis to three 

 weeks after it has occurred. 



