THE PINEAL GLAND 289 



Oppel's Lehrbuch der vergleichenden mikroskopischen Anatomie 

 der Wirbclthiere (1905). 



It is only within the last few years that our knowledge of 

 the fore-brain and its derivatives has advanced sufficiently to 

 enable us to formulate with any degree of precision the morpho- 

 logical relationships of the pineal outgrowths. For this purpose 

 we may accept, at any rate provisionally, the neuromeric sub- 

 division of the brain which we owe mainly to the embryological 

 investigations of Locy and Hill. It appears tolerably certain 

 that the vertebrate brain is built up of no less than twelve 

 neuromeres (fig. 1, N. 1-12), which can be recognised with 

 greater or less distinctness in the embryo and indicate the 

 primary metameric segmentation of this region. Of these 

 neuromeres the first three give rise to the fore-brain, the 

 fourth and fifth to the mid-brain, and the remainder to the 

 hind-brain. In the fore-brain the first neuromere (N. 1 ; Pros.) 

 gives rise to the prosencephalon ("telencephalon" of German 

 authors), from which the cerebral hemispheres (C.H.) are 

 developed, while the second and third {Thai) form the thalam- 

 encephalon (or diencephalon), with which alone we are at present 

 concerned. At a very early stage in development a pair of 

 outgrowths from the second neuromere form the optic vesicles 

 of the ordinary paired eyes, while the third neuromere probably 

 gives rise to the "pineal" or " epiphysial " outgrowths (Ep. 1, 

 Ep. 2), which appear at about the same time on the roof of the 

 thalamencephalon. In many fishes, and again in Lacertilia and 

 Sphenodon, it is easy to recognise two of these pineal outgrowths, 

 which may either lie one behind the other ab initio (e.g. in 

 Petromyzon), or may present more or less clear indications 

 of a bilaterally symmetrical arrangement (e.g. teleosts, Amia, 

 Sphenodon), suggesting a serial homology with the ordinary 

 optic vesicles. In the higher vertebrates one of these two 

 outgrowths is either completely suppressed or merged in the 

 other at a very early date, so that the " epiphysis " or " pineal 

 gland " may appear to be single and median even in origin. 



Immediately behind the fore-brain the anterior limit of the 

 mid-brain is marked by the posterior commissure (C.P.), which 

 lies just behind the pineal outgrowths, and at a short distance 

 in front of this lies the superior or habenular commissure 

 (C.H.S.), a transverse band of fibres which connects together 

 the two habenular ganglia lying on the optic thalami, and 



