6 E. R. HOSKINS AND M. M. HOSKINS 



region. In some cases, however, some of the muscles failed to 

 develop properly, and this has an important effect on the growth 

 of the animal. This will be discussed later. 



Among the frog larvae from which the thyroid had been re- 

 moved a few animals developed abnormal external gills, the 

 shape of which was variable. Some of these were low and 

 flattened, some long and slender, but irregular, and with a vari- 

 able number of filaments. In other specimens the filaments 

 were club-shaped being enlarged at the distal ends. The circu- 

 lation through these abnormal gills was often sluggish. One 

 experimental frog larva did not develop any external gills, al- 

 though it lived and grew otherwise apparently normally through 

 the period in which external gills normally persist. These bran- 

 chial malformations were considered to be the result of vascular 

 injury inflicted during the operation. Ekman ('13) states that 

 the blood-vessels are necessary for the later development of the 

 external gills. 



The twisting of the tail which is conmion among frog larvae 

 kept in aquaria occurred in our 1917 specimens which were not 

 under optimum conditions. It was also noted in the regeneration 

 experiment described below. It did not occur at all in the rapidly 

 growing 1918 larvae. The abnormality is not due to hardness of 

 water, but to food conditions, for the same kind of water was 

 used in both years. 



Growth. In our 1916 experiments it appeared that the average 

 growth rate was less in the operated animals than in the controls, 

 and it was so reported (Hoskins and Morris, '17). The reason 

 for this difference in the average rate of growth we later dis- 

 covered to be due to two chief causes, and the real difference 

 is just the opposite from what it appeared to be. In some cases 

 the operation injures the developing lower jaw and its muscles, 

 especially the submaxillary; and these animals have difficulty 

 in eating, and suffer from partial or complete inanition. In 

 other animals the thyroid is not completely removed, and re- 

 generates, and these animals grow at about the same rate as 

 the controls. The experimental larvae may be divided into 

 three groups: those with defective jaws or muscles which grow 



