64 Journal of Comparative Neurology. 



state that its appearance is frequently delayed beyond this 

 stage and I have observed hemispheres well on in the third 

 month with hardly a trace of it." What is later to become the 

 Sylvian fissure is now only a broad shallow furrow deepest near 

 the base of the brain and radiating in a fan-shape dorsad where 

 it disappears upon the lateral surface. Gradually by the 

 growth of what will later form the frontal, temporal and pari- 

 etal lobes, an overlapping or folding occurs and the fossa grad- 

 ually narrows until completely closed. What really happens 

 is that by the more rapid enlargement of these lobes in an en- 

 closed space, the intervening broad area in the bottom of the 

 fossa, which early becomes fixed in position, is finally covered 

 and the lips of the Sylvian fissure are brought together. These 

 overlapping portions are the so-called operculums and in the 

 human brain are four in number, namely : the opcradmn arising 

 from the frontal and parietal, the piropercnhim and the siib- 

 operculum from the frontal, and the postopcnuhtin from the 

 temporal lobe. These overlapping portions do not differ struc- 

 turally from the surrounding parts, being convoluted and cov- 

 ered with cinerea on both the ectal and the ental surfaces, and 

 the gyres of the ental surface are interdigitated with those of the 

 insula. The dorso-caudal end of the Sylvian fissure is the part 

 first formed and the first as well to become completely closed. 

 This is due to the rapid growth of the operculum and postoper- 

 culum. 



In 1890 Beer (1,309 (ix) ) said: " According to Ecker, 

 Mihalkovics, etc., the Sylvian fissure of the adult is formed by 

 the gradual narrowing of a flat broad depression on the lateral 

 surface of the hemisphere, which is due to arrest of develop- 

 ment of the ifistda Reilii in the 3rd month of fetal life." The 

 conclusion that the insular area becomes depressed by a cessa- 

 tion of growth, while favored by many neurologists, appears to 

 me unwarranted. Presumably what happens as stated by 

 Wilder is that there is probably no complete arrest of develop- 

 ment, but that the surrounding parts develop much more rap. 

 idly so that in a short time they project farther laterad, leaving 

 the insula relatively depressed. While this seems to account 



