REMOVAL OF NASAL PITS AMBLYSTOMA EMBRYOS 47 



It becomes apparent then that the absence of the nasal sac 

 has here resulted m a reduction in the size of the telencephalon 

 of the operated side, the reduction being limited to the anterior 

 portion of the hemisphere. 



It is perhaps best at this point to mention one of the results 

 of the operations on the histogenesis of the brain that has been 

 brought out by the technique employed in studying the gross 

 morphology. It is a well known fact that the growth of the 

 olfactory nerve is centripetal, — from the olfactory placode to the 

 telencephalon. The terminations of the ingrowing nerve fibers 

 are known as glomeruli. As would be expected, the absence of 

 the olfactory nerve results in the complete disappearance of the 

 glomeruli. So that there are present in the telencephalon of 

 the operated side only secondary and tertiary tract fibers, the 

 primary fibers being entirely absent. 



The material so far studied has as yet shown no discernible 

 effects on these secondary and tertiary tracts. Gudden's atrophy 

 is necessarily a slow process, and it is evident from the material 

 at hand that if they are to show this type of atrophy the oper- 

 ated larvae must be kept at least to metamorphosis. It is prob- 

 ably, too, that the atrophy would continue with age so that 

 metamorphosed larvae would show the most marked differences. 



A study of entire brains dissected out of normal and oper- 

 ated larvae, together with a careful inspection of transections of 

 the brains of parallel stages shows that the reduction in size is 

 not immediate. The first four weeks of development after the 

 operation show no differences in the size of the telencephalon. 

 That is, the growth of the two hemispheres during the first part 

 of larval life, three weeks of which have been dependent on yolk 

 and the fourth on the food in the environment, has not been in 

 the least affected by the absence of the nasal sac. From the 

 fourth week on, however, the hemisphere of the unoperated side 

 begins to grow more rapidly than its fellow of the operated side, 

 finally outdistancing it to the extent shown in the drawing of 

 the model. 



The fact that the difference in growth did not occur until 

 active feeding had commenced suggested at once that here was 



