Optic Lobes of Fowl. 81 



well marked, though the diencephalon is noAv becoming rela- 

 tively smaller than either the telencephalon or the mesence- 

 phalon. The first traces of the future cereljelhun are not visible 

 from the dorsal aspect, the swelling of the optic lobes posteriori}' 

 preventing this. 



In the lateral vieAv (2a) the rounded, more or less definite 

 form of the cerebral hemispheres is seen beginning to encroacli 

 on the diencephalon from the anterior end, while at the same 

 time the optic lobes are beginning to overgrow it from the- 

 posterior. The epiphysis is very definite on the dorsal surface, 

 and the optic chiasm iufundibulum are beginning to 

 appear ventrally. The metencephalon, showing traces of the 

 upwards and outwards growth which later form the ceiel^ellum, 

 is visible below the optic lobes. The angle formed by the axes 

 of the telencephalon, diencephalon and mesencephalon, and of 

 the mesencephalon, metencephalon and myelencephalon, is still 

 rather less than a right angle, but is not so acute as in stage I. 



2c (Fig. 2, Section 1) is a transverse section across the an- 

 terior part of the optic lobes. The lobes are now seen to be 

 lateral in position, and the walls are becoming slightly thicker, 

 while the optocoele is beginning to become smaller. 



In 2d (Fig. 2, Section 2) a section is taken across the middle 

 of the optic lobes, the cranial flexure still being so great as to 

 cause the metencephalon and myelencephalon to appear as a 

 longitudinal section. 



2e (Fig. 2, Section 3) cuts through the posterior portion 

 of the optic lobes, and the first beginnings of the cerebellum. 



Stage III. (PL XXVII.) 



The third stage corresponds approximately to a period of 

 nine days' incubation. The mesencephalon is now divided into 

 two rounded, dorso-laterally placed structures, which are. from 

 this time on called the optic lobes. They are still very large, 

 comparatively, and form the most prominent portion of the 

 brain. By this time the shape of the cerebral hemispheres has 

 become quite definite and characteristic, and the olfactory nerve 

 has begun to become apparent at the anterior end. 



