Clark, Comparative Anatomy of tJie Insula. Gy 



lutely overlap each other. These, with the possible exception 

 of the Australian, are all types of individual difference and their 

 significance may be summed up thus : (i) if the insula is not 

 wholly concealed, it shows that if the insula is normal, the sur- 

 rounding parts have not reached their full development, while 

 if the insula is abnormally large, the surrounding parts may or 

 may not be fully developed ; {2) if the insula is completely 

 covered so that the operculums overlap each other, it would 

 appear that, if the insula is normal, the surrounding gyri must 

 be abnormally developed, while if the insula were unusually 

 small, the normal operculums might even overlap each other. 



EARLY DEVELOPMENT OF THE HUMAN INSULA. 



We have seen that the rudiments of the Sylvian fissure 

 become apparent toward the end of the second month of 

 fetal life. There is then in the bottom of the shallow fossa 

 an indefinitely-defined oval or somewhat triangular area which 

 apparently develops more slowly than the surrounding lobes 

 and which finally becomes a subarea. As development pro- 

 ceeds, this area becomes more elevated and hence more sharply 

 defined by the formation of the circuminsular fissure. The re- 

 gion thus mapped out is sometimes called the insular area be- 

 cause it is the part which will eventually become the insula in 

 man and presents a condition found in the adults of some 

 animals. 



Mingazzini (40, 217, 1888) says: "Until the end of the 

 sixth month the insula is quite smooth ; constantly there ap- 

 pear in the seventh month those fissures [ circuminsular, 

 Wilder ; rigolcs, Broca] which circumscribe the future gyres 

 of the insula. I can therefore not agree with Mikalkovics who 

 asserts that the insula is smooth up to the beginning of the ninth 

 month. I found this quite so once in a brain of the eighth 

 month upon which only one small cortical fissure appeared. 

 In the eighth month, they increase, oftentimes, to the number 

 of four, among which is produced the caudal one which is larger 

 and deeper than the others ; after the ninth month, there are, 

 as a rule, four fissures." Mingazzini probably erred in the 



