NERVOUS SYSTEM. VERTEBRATA. 365 



sulcus on the right hemisphere (figs. 206 and 207), whereas 

 the lower remains entirely independent (fig. 207, e). 



Such an interpretation, however, is not wholly correct 

 because the resemblance to the Carnivore condition is 

 largely spurious : it is not the persistence in a Primate of 

 the common mammalian plan of sulci, but a special com- 

 bination produced by profound retrogressive changes oc- 

 curring in a distinctly and characteristically-Primate brain. 

 In most of the Lemuroidea the coronal sulcus of the 

 Ungulates and of the more generalised members of other 

 Orders (the conjoint proreo-coronal sulcus of the Rodentia, 

 or the union of the separated coronal and prorean elements 

 of the Carnivora, Edentata, and Marsupialia) persists in a 

 practically unchanged form. In the series of retrogressive 

 modifications which the primitive Prosimian brain must 

 have undergone to produce the condition found in the 

 Aye-aye, the region surrounding the coronal sulcus has 

 suffered most change, and, as these four hemispheres 

 abundantly show, exhibits the greatest amount of varia- 

 bility. The correct interpretation is a matter of extreme 

 difficulty ; but a thorough study of the Prosimian brain 

 leads me to the opinion that the coronal sulcus becomes 

 broken up into two irregular fragments in Daubentonia : 

 the anterior of these (77) usually joins the orbital sulcus. 

 The posterior (e) exhibits great irregularity in shape and 

 in behaviour. It may join a furrow () which probably 

 represents the crucial sulcus of other mammals, and the 

 result is the furrow e 4 ?, which I believe to be identical 

 with the sulcus centralis of the Primates. The furrows e 

 and may remain separate and may be linked to the 

 lateral sulcus (fig. 207). [These views are put forward 

 chiefly as the result of a study of the brain in the Indri- 

 sinse, which are the nearest relations of the Aye-aye.] 



Most of the other parts of the brain conform to the type 

 presented by the great majority of mammals. But, in 

 accordance with the relatively large dimensions of the 

 cerebral hemispheres, the crura cerebri and pyramidal 

 tracts are large and prominent, as in all the Primates. 

 Moreover a definite olivary body is present. 



