THYROID REMOVAL IN LARVAE OF RANA PIPIENS 507 



It is my intention to follow out a series of experiments to test 

 the ability of the tadpoles to respond to the stimulus of thyroid 

 feeding at various periods after the cessation of development. 

 This should give us an insight into this very interesting phase of 

 the subject of senescence. 



Thyroid removal has a decided effect upon the growth of the 

 brain. This is evident in many features notably as regards the 

 cerebral hemispheres and the optic lobes. A noticeable effect 

 is seen in the development of the cerebellum. The figures speak 

 for themselves. Figure 9 shows the brain of a normal tadpole 

 that has slightly passed the stage at which the thyroidless tad- 

 poles ceased to develop. The total length is 62 mm. and the 

 body length 27 mm. This might well be compared with figure 10, 

 that represents the brain of a tadpole at metamorphosis — body 

 length 24 mm. While the total length of the brain in this case is 

 no greater than in the foregoing, it may be seen that the cerebral 

 hemispheres have undergone a change of form being broadened 

 posteriorly and far more sharply cut off from one another than 

 before. The marked broadening of the optic lobes is especially 

 striking. An increase in the width of the diencephalon is to be 

 seen. With these processes the cerebral hemisphere and the 

 optic lobes have more closely approached one another. Figure 

 11 shows the brain of a young normal frog of 50 mm. body length 

 killed February 22. This is less than a year old, having un- 

 doubtedly metamorphosed the preceding summer. It shows 

 little change in form and proportion over the brain at metamor- 

 phosis. The transverse furrow in the cerebral hemispheres is well 

 marked. These have grown backwards over the diencephalon 

 coming almost in contact with the optic lobes. The cerebellum 

 shows a distinct increase in size approximating its final 

 proportions. 



Now with this normal development of the brain in mind, it is 

 especially interesting to study the characteristics of the brain 

 of a thyroidless tadpole killed February 7 with a length of 123 

 mm. and a body length of 43 mm. (fig. 12). One is immediately 

 struck with the immaturity of this brain. This is notably true of 

 the cerebral hemispheres and the optic lobes. These show but 



