568 LOCV. [Vol. XI. 



Figs. 43, 44, and 45, which are successively older, show an 

 increase in the size of the thalamencephalon, and marked 

 changes in the mid-brain. 



In Figs. 46 and 47 a faint furrow has appeared in front that 

 serves to mark the boundary between the thalamencephalon 

 proper and the cerebral lobes. This furrow is clearly defined 

 in Figs. 48 and 49, so that we have now the thalamencephalon 

 distinctly marked off from the rest of the fore-brain. 



In Fig. 48 the optic vesicle and all the mesoblast have been 

 removed, and the view is taken from the right side. The brain- 

 walls show very clearly. The thalamencephalon is now bounded 

 anteriorly and posteriorly by furrows. The two furrows run 

 nearly parallel to one another, and they serve to mark off the 

 inter-brain very distinctly. 



The roof of the thalamencephalon is at this stage raised into 

 two rounded eminences of nearly equal dimensions, an anterior 

 and a posterior one. The posterior is, from the beginning, 

 somewhat in advance of the other. It becomes the pineal out- 

 growth, while the posterior eminence goes on developing. The 

 anterior one becomes greatly reduced by compression from the 

 rapidly growing adjacent brain-walls. It very soon loses its 

 rounded character, and becomes pressed into a semicircular 

 fold lying in front of the epiphysis. It is, I think, equivalent 

 to the Zirbelpolster of German authors. If longitudinal sec- 

 tions be made at the stage represented in Fig. 48, the two 

 elevations show as in Minot's Fig. 319, p. 572, of his Human 

 Embryology. In that figure they look like two equal vesicles 

 springing from a single elevation of the brain roof, and open- 

 ing by a common wide passage into the brain vesicle. No 

 especial significance, I think, is to be attached to the fact that 

 there are at first two equal embossments from the roof of the 

 thalamencephalon. The anterior one is converted into a fold 

 of the roof of the 'tween-brain that has long been recognized in 

 different animals. It is designated Zirbelpolster by Burckhardt. 

 It does not in any sense correspond to the anterior epiphysial 

 vesicle discovered by Hill in Teleosts, but rather to the fold 

 of 'tween-brain that lies in front of both epiphyses. Studnicka 

 has, however, marked the fold in diagrammatic figures of Tele- 



