Clark, Comparative Anatomy of the Insula. 65 



for all observed phenomena, the writer is inclined to the belief 

 that one very important factor has been underestimated. Dur- 

 ing the period when the insula first appears, the brain as a 

 whole is enlarging very rapidly. This enlargement in the re- 

 gion which will later become the Sylvian fissure it is assumed 

 may be due more largely to the expansion of the paraccele 

 than to the thickening of the paracoelian wall by the addition 

 of true nervous substance, which of course is always interstitial. 

 The anatomy of this region would seem to favor this latter 

 conclusion. The intimate connection of the insula with the 

 brain stem is such as to preclude any marked lateral extension 

 and, as the wall of the paraccele is then but little more than 

 membrane, any sudden increase of development, either by add- 

 ition to the true nervous parietes or by the expansion of the 

 paraccele, would cause the area caudad and dorsad to project 

 laterad giving outline to the insula by the formation of the cir- 

 cuminsular fissure and later the dorsal end of the Sylvian. It 

 also suggests the possibility of finding in a fetus of the 

 fourth to the seventh month an expansion of the paraccele 

 corresponding in a general way to the outline of the rudimen- 

 tary operculum. The conclusion given above agrees with ob- 

 servations upon the development of this region as described by 

 Cunningham, Ecker and others. It is also further believed that 

 this effect of expansion does not stop here, but that the over- 

 lapping of the insula and consequent formation of operculums 

 may be due in part to the expansion of the paraccele, and that 

 the subsequent thickening of the walls by the addition of true 

 nervous substance combined with the effects of growth in an 

 enclosed space may account in part for the continued conceal- 

 ment of the insular area in the adults of Primates and some 

 of the Carnivora. 



The exact time of the closing of the Sylvian fissure by the 

 operculums is not known. In 1873, Ecker said it was from 

 the ninth to the tenth month. 



In 1888 Mingazzini (41) said: "In one brain of the 

 eighth month, I have found the fissure almost completely 

 closed." The condition here presented I think is one of indi- 



