optic Lobes of Foni. 83 



such a way as to hide it except at the median dorsal surface. 

 The epiphysis is distinctly visible in the centre of this space. 

 The optic lobes have encroached on the diencephalon, so that 

 now they are closely approximated to the cerebral hemispheres. 

 The pressure between the two portions of the brain is such that 

 the optic lobes are beginnino; to be pushed out laterally, and 

 their axis in the longitudinal vertical plane tends to become 

 placed obliquely to that of the cerebral hemispheres and 

 diencephalon in the same plane. The outgrowth of the metence- 

 phalon, to form the cerebellum, is now becoming visible between 

 Ihe posterior portions of the optic lobes. 



In 4a the backward growth of the cerebral hemispheres and 

 the forward growth of the optic lobes, so as to be in close 

 approximation with each other, is clearly indicated. The optic 

 "chiasma and the infundibulum are distinct, and the ventro- 

 lateral depression of the cerebral hemispheres, caused by the 

 formation of the floor of the orbit, is becoming more clearly 

 marked. The epiphysis, still comparatively large, is seen in 

 profile. The rotation of the axis of the optic lobes is not so 

 ■easily discernible in the lateral view. The fold of the metence- 

 phalon, destined to become the cerebellum, is assuming a more 

 definite shape, and the angle formed by the axis of the cerebral 

 hemispheres and the optic lobes in longitudinal horizontal plane, 

 and the axis of the optic lobes, and metencephalon, and 

 myelencephalon in the same plane, is becoming much more 

 •obtuse, it now being somewhat less than a right angle. 



4c (Fig. 1, Section 1) is taken across the anterior part of the 

 optic lobes, and includes the epiphysis on the dorsal surface, 

 and the metencephalon and myelencephalon in the longitudinal 

 section. 



The walls of the optic lobes are becoming much thicker now, 

 and the histological structure is commencino- to be differentiated. 



4d (Fig. 4, Section 2) is a section across the centre of the 

 -optlo lobes. 



The crura cerebri are more definite than in the preceding 

 figures, and the myelencephalon is seen in longitudinal section. 



4e (Fig. 4, Section 3) shows the posterior portion of the optic 

 lobes in section. They are thicker-walled than in the preceding 

 stage, and their histological structure is more definite. 



