438 J. GRAHAM KHJKK. 



Mesencephalon. 



Of the mid-brain there is little to be said. The roof 

 remains throughout development thin in the mesial plane, 

 except anteriorly, where it thickens out as it approaches the 

 posterior commissure. 



The floor of the mid-brain shows anteriorly very distinctly 

 the two median pits which it has been suggested indicate the 

 tAvo anterior segments of the mid-brain. These depressions 

 are seen in the series of sagittal sections (Text-fig. 2, F — H) : 

 they first appear about Stage 35 as slight crinklings of the 

 posterior wall of the infundibular depression. 



Burckhardt points out, in regard to Pro top terns, that 

 the floor of the mid-brain is bounded posteriorly by inpush- 

 ings of the two limiting membranes* The dorsal one of these 

 becomes very sharply marked in Lepidosiren about Stage 

 35. It increases in depth for a time, but then flattens out 

 again, and in Stage 39 has become replaced by a slight 

 elevation of the surface. There appears to be no trace of the 

 lower groove. 



Rhombencephalon. 



The cerebellum becomes evident about Stage 31 as a slight 

 thickening of the roof of the fourth ventricle. The thicken- 

 ing is much more conspicuous laterally than in the middle 

 line. 



Behind this the roof of the ventricle at an early stage 

 becomes thin and membranous. 



Olfactory Organ. 



The olfactory organ ni-ises from a solid ingrowth of the 

 deej) layer of the ectoderiu. 



Within the few days after hatching (Stage 29 +) the 

 cavit}' apjiears in the riuliiiicut first in the form of n s]ilit. 



