II 



BEAIN, 



87 



groove bounded behind and in front by a slightly projecting ridge— 

 the rudiments of the optic chiasma and of the anterior commissure 

 respectively (Fig. 53, eh, a.c). 



About stage 31 a little pocket-like diverticulum of the roof 

 of the primitive fore-brain makes its appearance (Fig. 53, D, pin). 

 This is the pineal body and its appearance is of topographical 

 importance as serving to demarcate the primitive fore-brain roof 

 into thalamencephalic and mesencephalic portions. 



Fig. 52. — Brain of young Lepidosiren as seen from the left side. 



A, stage 26; B, stage 31; C, stage 35- ; D, stage 39. c.H, cerebral hemisphere; /, primitive fold 

 of brain- floor; inf, infnndibulum ; o.b, olfactory bulb; o.t, olfactory tubercle; pin, pineal body; 

 thai, thalamencephalon ; t.o, tectum opticum. 



The lateral bulgings of the fore-brain have become more pro- 

 minent and now project forwards beyond the limit of the rest of the 

 fore-brain. In the mesial plane between the two hemispheres there 

 projects upwards and forwards a little pocket of the anterior wall 

 of the fore-brain. This is the rudiment of an organ of unknown 

 significance — the paraphysis (Fig. 53, D, par). 



Soon after the appearance of the pineal body the roof of the' 

 primitive fore-brain becomes divided into a posterior portion belong- 

 ing to the mesencephalon and an anterior portion belonging to the 

 thalamencephalon (Fig. 51, B, t.o, and thai). As development goes 



