82 K K. Oliver: 



In 3b the cerebral hemispheres can be seen to be becoming 

 elongated, and are stretching backward to meet the forward 

 growing optic lobes. The epiphysis is prominent in the space 

 still existing between the two pairs of structures, which is all 

 that is visible of the diencephalon from the dorsal aspect. The 

 forming cerebellum is hidden by the backward growth of the 

 optic lobes. 



In 3a the approximation of the cerebral hemispheres to the 

 optic lobes is very marked in comparison with the preceding 

 stage. From the lateral position the only part of the dience- 

 phalon visible is the epiphysis on the dorsal, and the infundi- 

 biihim and optic chiasma on the ventral surface. Tlie ventro- 

 lateral depression, caused by their position against the back of 

 the orbit, is now becoming visible in the cerebral hemispheres, 

 and the olfactory nerve is also to be seen forming. The optio 

 lobes have grown forward in such a way as to overshadow the 

 diencephalon from behind and the metencephalon from above. 

 The lateral enlargement of the metencephalon, marking the 

 commencement of the cerebellum, is becoming more clearly 

 indicated. 



3c is a transverse section in the plane indicated in Fig. 3, 

 Section 1. It shows the increase of the brain substance over the 

 preceding stage, and the thickening of the Avails of the optic 

 lobes. 



In 3d the lateral position of the optic lobes is very definite. 

 The crura cerebri are faintly indicated at this stage. 



3e is a transverse section across the hinder part of the optio 

 lobes, and just touches the myelencephalon longitudinally. 



The axis of the cerebral hemispheres and the optic lobes, and 

 that of the optic lobes and the metencephalon and myelen- 

 cephalon have now altered, so as to become approximately at 

 rio^ht ancrles to one another. 



'&•■ 



Stage IV. (PI. XXVIII.) 



The fourth stage corresponds approximately with a period of 

 10 days' incubation. In the dorsal view (4b) the cerebral 

 hemispheres can be seen as elongated, closely opposed struc- 

 tures. Laterally they have encroached on the diencephalon in 



