508 



COMPAEATIVE ANATOMY. 



Reptiles. The most important changes are seen in the prosen- 

 cephalon, which has the olfactory lohes, -which are as a rule still 

 hollow, connected with its lower surface ; these lobes are more or 

 less covered over by the hemispheres, accordiug to the extent to 

 which they are developed. The prosencephalic hemispheres are 

 always separated by a fissure, which is very deep anteriorly. They 

 are primitively connected together by a commissure which is placed 

 in front of the primitive cerebral cleft ; through the cleft there is a 

 passage into the cavities of the prosencephalon, or lateral ventricles. 

 When they are more highly developed, the hinder portions of the 

 hemispheres increase in size; the cleft, which was at first small, is 

 laterally extended and disappears from the surface, being completely 



A 



B 



Fig. 28G. Differentiation of the proscnceptalon. A Brain of a Cliclouian, B Of 

 a foetal Calf, C Of a Cat. In A and B the roof of the prosencephalic cavity is 

 removed from the left side, and the fornix from the right. In C the whole of the 

 lateral and posterior portions of the right prosencephalic lobe are removed, and so 

 much on the left as is necessary to display the upward bend of the cornu ammonis. 

 In all the figures I marks the prosencephalon. II Thalamencephalou. Ill Mesence- 

 phalon. IV Metencephalon. V Myelencephalon. ol Olfactory bulb (shown in A as 

 communicating with the cavity of the prosencephalon), si Corpus striatum. / Fornix. 

 h Pes hippocampi major, s r Rhomboidal sinus, cj Geniculate process. 



covered over by the hinder wall of the lateral ventricles, which have 

 grown out at the sides and behind. With this is correlated the differ- 

 entiation of the primitive commissure into a system of commissures, 

 the lowest stage of which is represented in the Monotremata and 

 Marsupialia. The pi'imitive commissure is first differentiated into 

 an inferior and a superior portion ; the former is the anterior com- 

 missure ; the latter forms a slender bridge, which is placed above the 

 anterior edge of the thalamencephalou, and below which there is on 

 either side the entrance into the lateral ventricles ; these last extend 

 downwards and backwards. In their anterior region the corpus 

 striatum forms a projection (Fig. 286, i> (7 st), and posteriorly there 

 is a rounded process, which is connected with the upper portion of 



