340 DEVELOPMENT OF THE HUMAN BRAIN 



A rudimentary ' fissure of Sylvius ' is to be seen on. the 

 outer surface of each, and from this wide and deep sulcus 

 a number of shallow fissures have been described by 

 Gratiolet and others (corresponding with internal promi- 

 nences on the walls of the lateral ventricles). These 

 appearances are believed by some to be artificial ; but 

 whether artificial or natural, all are agreed that they dis- 

 appear after a time, as the walls of the ' lateral ventricles ' 

 become thicker. Then it is that the permanent ' fissures' 

 and ' convolutions' begin to be developed on the external 

 surface of the Cerebral Hemispheres. 



At this period, too, the Corpora Striata are distinctly 



FIG. 127. Brain of Turtle (Chelone\ side view, for comparison with last figure. 

 (Owen.) C, Cerebellum ; O, optic lobes ; P, Cerebrum ; R, olfactory lobes. 



larger, and not far from their anterior extremities a short 

 and nearly vertical Corpus Callosum is recognizable (not 

 very different from that which exists in Marsupials). 

 The Anterior Commissure is slender but distinct. The 

 Middle or Soft Commissure exists in the form of a 

 large rounded projection from the inner face of each 

 Thalamus, though the two prominences have not yet come 

 into contact with one another, so as to form an actual 

 Commissure. 



The cavity within the Optic Lobes is even larger than 

 it was at an earlier date. The lateral lobes of the Cere- 

 bellum have developed notably, whilst they are separated 

 from one another (fig. 126, c) by a median depression 



