THE PINEAL ORGAN OF MAMMALS 



309 



ward on each side of the main pineal diverticulum, A, Ep. D. There is 

 also a slight indication of the development of the anterior lobe, which 

 appears as two solid epithelial buds from the anterior wall of the pineal 

 diverticulum, B, A.L. An infrapineal recess was also present in this 

 specimen, D, I P.R., which probably represents a temporary fold and 

 would have disappeared at a later stage of development, when the apex 

 of the pineal organ becomes rotated backward and comes to lie on the 

 dorsal aspect of the quadrigeminal plate. 



In a 6-cm. human embryo, the anterior lobe has considerably increased 

 in size and a secondary pineal stalk has developed and forms rather more 

 than one half of the total outgrowth, the distal end of which consists of 

 the main pineal diverticulum (Fig. 213, A, P.O., and B, P.D.) and a 

 smaller posterior diverticulum (Fig. 213, B, Post. ZX), which lies imme- 

 diately above the posterior commissure. The anterior lobe consists of a 

 number of irregularly branched processes of epithelium, which grow 

 into the vascular mesenchyme in front of the main pineal diverticulum 

 and above the habenular commissure. Similar proliferating outgrowths 

 of ependymal cells from the sides of the main diverticulum are seen invad- 

 ing the mantle zone ; these, however, lie beneath the external limiting 

 membrane Fig. 214, A, A.L., Ep. C, and have not, as yet, come into contact 

 with the blood-vessels of the surrounding mesenchyme. Two diverticula 

 were present in front of the pineal organ ; the more posterior of these 

 probably represents the diverticulum from the dorsal sac, which will 

 become the suprapineal recess (Fig. 213, B, D.D.) ; the more anterior 

 is possibly a rudiment of the paraphysis {Pa.). The close relation of the 

 great cerebral vein to the epiphysis is indicated in the lineal reconstruc- 

 tion (Fig. 213, B, G.C.V.) and also the position of the epidermis, which at 

 this stage is raised a considerable distance above the pineal organ. The 

 exact position relative to the membranes of the brain is shown in Fig. 213, 

 A, P.O. The organ lies in a triangular space bounded below by the roof 

 plate of the neural tube and the layer of pia mater which invests the 

 brain stem, and laterally by right and left membranous lamina;, which 

 are attached above to the lower border of the interhemispheric septum 

 or primary falx cerebri. Along the line of junction of the lateral laminee 

 with the interhemispheric septum is a membranous channel which encloses 

 the great cerebral vein (Fig. 215, G.C.V. ). At a later stage of develop- 

 ment, when the corpus callosum and fornix grow backward over the 



