GENERAL CONCLUSIONS 475 



differences which serve to place the ostracoderms definitely among the 

 fishes, and not, as was formerly supposed, in an intermediate position 

 within the direct line of descent of the vertebrates from a highly differ- 

 entiated ancestral arthropod, such as Limulus, or a species resembling 

 any other of the living arachnids. 



Summary of Observation on the Development and Structure of the 

 Human Pineal Organ 



1. The pineal diverticulum first appears in human embryos of approxi- 

 mately 15 mm. length. 



2. The apex of the diverticulum is primarily directed forwards. 



3. The pineal outgrowth lies a short distance in front of the posterior 

 commissure, and sometimes presents a constriction subdividing it into 

 an anterior and posterior segment. 



4. The whole thickness of the neural wall participates in the forma- 

 tion of the pineal evagination. 



5. In some specimens there is an indication of the anterior segment 

 being subdivided into right and left lobes. 



6. The " anterior lobe " first described by Krabbe appears in embryos 

 of about 22 mm. length as several neuro-epithelial buds which grow for- 

 ward into the surrounding connective tissue. 



7. A well-marked supra-pineal recess (dorsal sac) is present at the 

 22-mm. stage. 



8. Transverse grooves, which are produced by folding of the roof 

 of the aqueductus cerebri in the region of the posterior commissure, 

 represent temporary infrapineal recesses. 



9. Between the third and fourth months of foetal life there occurs 

 an active proliferation of cells derived from the inner or ependymal 

 zone of the pineal diverticulum. These grow outward in the form of 

 cords, the component cells of which are arranged radially round a central 

 axis which is destitute of nuclei. This is accompanied by a simultaneous 

 ingrowth of vascular processes of mesenchyme. 



10. A special mass of proliferating cells growing from the anterior 

 wall of the main diverticulum gives rise to the solid anterior lobe of 

 Krabbe, whereas the cords which grow from the fundus of the diverti- 

 culum form the principal solid part of the posterior lobe. The cavities 

 at the base of the stalk and that of the posterior diverticulum appear to 

 open out, and their lumina thus become incorporated in the cavity of 

 the third ventricle, whereas the cavity of the main or anterior diverticulum, 

 which may be cut off as the " cavum pineale," usually disappears. 



11. The neuro-epithelial cells give rise to (1) the glia lining the fibrous 

 capsule and covering the trabecular, (2) the parenchyma cells, and (3) the 



