80 K. K. Oliver: 



phalon is worth remark. BetAveen the mesencephalon and 

 metencephalon the isthmus is \%yj distinctly marked. At this 

 stage the line of axis of the mesencephalon and the metence- 

 phalon, and the line of axis of the cerebral hemispheres and the 

 mesencephalon form an acute angle. 



Ic (Fig. 1, Section 1) shows a transverse section through the 

 anterior portion of the mesencephalon. The walls of the 

 mesencephalon are thin, and the lumen of the cranial cavity 

 great in comparison with the thickness of the cranial wall. 



Id (Fig. 1, Section 2J shows a transverse section through the 

 centre of the mesencephalon and metencephalon. The section 

 owes its apparently -continuous shape to the fact that, because 

 of the flexure of the brain, the section through the mesence- 

 phalon is transverse, and that through the metencephalon 

 lonoitudinal. 



le (Fig. 1, Section 3) shows the posterior portion of the 

 mesencephalon. The slight groove marking the beginning of 

 the optic lobes is faintly indicated. 



Stage li. (PI. XXYI). 



This stage corresponds approximately with the jDeriod of five 

 days' incubation. At this stage the mesencephalon has become 

 very definitely divided in the sagittal plane, the groove shown 

 in stage I. having deepened in such a way as to indicate the 

 outlines of the optic lobes. Although further development has 

 taken place, these have not increased in size as much as the 

 cerebral hemispheres, w^hich are now assuming a definite form, 

 and are enlarging considerably. This growth brings the cerebral 

 hemispheres further baick, towards the optic lobes, but there is 

 a distinct space still between them. 



The dorsal view (2b) shows the clear division of the mesence- 

 phalon into the two optic lobes, so that now, from being single 

 and median, the mesencephalon has become a paired lateral 

 structure, which is still very prominent on the dorsal surface. 

 The cerebral hemispheres have increased in size at a rate which 

 is out of proportion to the rate of growth of the optic lobes, 

 which, though undergoing further development, have not mate- 

 rially enlarged since the preceding stage. The epiphysis is still 



