Optic Lobes of Foivl. 85 



the cerebral hemispheres on either side, and the optic chiasma 

 and infundibulum are less distinct. The ventro-lateral depres- 

 sions of the cerebral hemispheres are at this stage very distinct, 

 there being a hemispherical concavity on each side of this por- 

 tion of the brain. The characteristic backwards curling cere- 

 bellum, with its faintly marked convolutions, is a marked feature 

 of this stage, and the long axes of the fore, mid and hind brain 

 now lie in the same general direction. 



In 5c (Fig. 5, Section 1) the increase of amount of the brain 

 substance is worthy of remark. The section is purely trans- 

 verse now, the change in the direction of the axis of the medulla 

 eliminating the necessity of cutting it longitudinally. 



5d (Fig. 5, Section 2) shows the lobes at about the centre. 

 The crura cerebri are very distinct now, and are beginning to 

 fill a considcTable portion of the cavity of the optocoele. 



A section across the posterior portion of the lobes in 5e (Fig. 

 5, Section 3) shows the increased thickness of the walls at this 

 stage. 



Stage VI. (PI. XXX.) 



Tlie sixth stage corresponds approximately to the- period of 

 thirteen days' incubation. 



The dorsal view shows little change since the last stage. The 

 cerebral hemispheres are becoming rather fuller and more 

 rounded in appearance, and are now well developed on the dorsal 

 surface. The epiphysis is distinct in the space between the 

 cerebral hemispheres and the optic lobes. The optic lobes 

 themselves are beginning to become more decidedly flattened 

 antero-dorsally, and have the appearance of being squeezed 

 between the cerebral hemispheres and the cerebellum in such a 

 way as to force the lobes down to a lateral and ventral position, 

 so that the greatest breadth of the brain is now that across the 

 optic lobes, and their long axis is at right angles to that of the 

 cerebral hemispheres and the cerebellum. The cerebellum has 

 developed considerably, and is now definitely convoluted, and 

 the floceuli are commencing to be formed laterally. The 

 myeleiicephalon has assumed the definite form of the medulla 

 oblongata. 



