GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 45 



do not grow to the size at which differentiation occurs. Their 

 growth tendency being weaker than that of some other tissues 

 (Jackson, '15), they grow less than these other tissues during 

 inanition. In this failure to grow they resemble many other 

 organs. 



Like the ovaries, the testes in these thyroidless larvae may 

 be said to be independent of the soma only in the same sense 

 that various parts of the soma are independent of the rest of 

 the body. Differentiation of the testes, as of the heart, eyes, 

 brain, and other organs, is a matter of growth after the anlage 

 is once laid down in the embryo, and in these thyroidless larvae 

 it must be remembered that the general growth process goes on 

 after maturation and after the testes have attained their mature 

 shape. Several organs differentiate before the gonads and others 

 later, so one cannot draw a sharp distinction between soma and 

 gonads, as Allen ('18) does, although the gonads do have some 

 individuality in their growth. Were the gonads entirely inde- 

 pendent of the soma, their differentiation in our 1917 animals 

 should have occurred much earlier than it did fin August and 

 September), since in the more rapidly growing 1918 larvae differ- 

 entiation of the gonads began in June and July. In attempting 

 to compare the testes of the thyroidless larvae and frogs Allen 

 ('18) makes the unfortunate mistake of trying to reduce figures 

 representing volumes of organs to relatively equal values by 

 dividing these volumes by the length of the animal. The error 

 of this method is discussed above. 



Microscopic structure. In none of the testes of control larvae, 

 or thyroidless larvae of the same size, or of young frogs was 

 synapsis seen to have occurred, so that in these groups of animals 

 the rate of differentiation of the gland could not be determined 

 (figs. 104 and 105). In thyroidless larvae which were the same 

 age as the controls (1917), but larger, synapsis was well under 

 way, and often nearly complete in August when spermatocytes 

 were found in the sections (fig. 107). In the 1918 (rapidly 

 growing) thyroidless larvae spermatogenesis was hastened, but 

 less than the growth of body as a whole. In these larvae synapsis 

 was observed in the testes in July, about a month after the con- 

 trols had metamorphosed. 



