84 



On the Morphology of the Pineal Eegion 



series the anterior end of the notochord is bent ventralwards at a 

 sharp angle, Fig. 2, nch; it tapers off gradually, and has a rounded 

 termination, which lies close against the wall of the praemandibular 

 cavity. The roof of the diencephalon has advanced slightly as com- 

 pared with figure 1. The velum, V, projects further into the cerebral 

 cavity and appears more clearly as a fold of the brain wall; in another 

 embryo of the same length, No. 235, section 63, the projection is more 

 marked. The epiphysal anlage has begun to deepen, Ep., and imme- 

 diately behind it the brain wall shows a thickening where the posterior 

 commissure is to appear. 



As shown in the figure the narrower diencephalon is marked ex- 

 ternally by a depression between the mesencephalon, M. B. and prosen- 

 cephalon, F. B. This depression can still be easily traced in embryos 

 of 40-45 mm, but gradually disappears. 



Fig. 3. Embryo of 1.5 mm. Sagittal series, No. 238, section 51. x 30 diams. 

 Fig. 4. Embryo of 18 mm. Sagittal series, No. 204, section 100. x 30 diams. 



In embryos of 15 mm.. Fig. 3, the epiphysal evagination, Ep., has 

 grown, as has also the velum, Y, which now has distinctly an anterior 

 and posterior surface; the ependymal coverings of the two surfaces lie 

 close together so that there remains only a minimal space between the 

 two ependymal layers. In this specimen the brain-wall is thicker in 

 the pineal region than in both the younger and the older stages, which 

 I have examined. Similarly the walls of the epiphysal anlage. Fig. 5, 

 Ep., are thicker than in both the older and younger stages of Figs. 4 and 

 6. I think these variations are not important morphologically. 



The next. Fig. 4, is from an embryo of 18 mm, and shows a decided 

 advance of the differentiation. The velum, Y, is now a well-marked 

 fold of the ependyma, with a narrow middle layer of mesenchyma; the 

 fold extends across the fore-brain from side to side, and according to 

 the current interpretation is regarded as marking the boundary be- 

 tween the diencephalon and the prosencephalon. The epiphysal anlage 

 is more distinctly an evagination than before. Immediately in front of 

 it has appeared the small but well-marked superior commissure, s. c, 



