508 BENNET M. ALLEN 



little advance in size and differentiation from the condition found 

 at the cessation of general bodily differentiation as shown in 

 text figure 2, B. The optic lobes have become more erect and 

 rounded; but there is little change elsewhere save in the marked 

 broadening of the diencephalon and myelencephalon. This how- 

 ever is in no sense proportional to the difference in body size. 

 Yet it is in clear contrast to the failure of the cerebral hemi- 

 spheres and optic lobes to appreciably increase in size. 



THE GONADS 



At the stage when the thyroidless tadpoles cease to undergo 

 bodily differentiation the gonads have already undergone sexual 

 differentiation. Specimens of this stage were not preserved; but 

 studies were made upon controls killed just before and at the 

 time of metamorphosis. These included the specimens given 

 in table 3 whose testes have the dimensions indicated. 



It is thus seen that the testes of the thyroidless tadpole killed 

 February 7 are many times as large as those of the control frogs 

 killed at the time of metamorphosis. In structure they are far 

 more advanced. This is especially true of the rete tubules 

 which- have joined with the ampullae. The latter have become 

 hollowed out in the thyroidless specimen of February 7 while they 

 are still solid in the normal controls killed at the time of meta- 

 morphosis. The most striking advance is in the development of 

 the germ-cells. In the controls at the time of metamorphosis 

 these are in the condition of spermatogonia. There are no 

 spermatocytes visible. In a thyroidless tadpole killed Septem- 

 ber 14 there are many cells in synapsis and one can find here and 

 there, clusters of spermatids. Careful search, however, did not 

 reveal any spermatozoa. It is unfortunate that there are no 

 controls killed at this time. In the thyroidless tadpole killed 

 September 25, there are some very clear clusters of spermatids. 

 The same conditions are found in the thyroidless specimen of 

 October 10. It is quite certain that no spermatozoa have been 

 formed as yet. The first clear instances of sperm were found 

 in the thyroidless tadpole killed December 15. At this stage, 

 they are still few in number, but in the one killed February 7, 



