312 THE PINEAL ORGAN 



(22-mm. human embryo). This pocket, which has been described as the 

 infrapineal recess, disappears at a later stage of development, probably 

 by opening out into the cavity of the third ventricle ; and it seems probable 

 that the cavity of the secondary pineal stalk (Fig. 213, B, P. St.) also 

 becomes absorbed into the ventricle in the same way, leaving only its 

 apical part, which persists in the adult as the definitive pineal recess. 

 The central cavity of the body of the main pineal diverticulum, which 

 according to Krabbe becomes closed by constriction at the neck, persists 

 for a variable time ; thus the cavity has usually disappeared at birth, 

 but the pineal body may be solid, with the exception of the pineal recess 

 at its base, at a much earlier stage, e.g. in a 40 -month human foetus, 

 which we shall describe next (Fig. 216, and Fig. 214, B and C). 



The mid-foetal stage of development of the pineal organ in the human 

 subject is very instructive. The anterior lobe, which is medial in position, 

 is seen in a coronal section to be partially separated from the main part 

 of the organ by two fibro-vascular septa, which pass obliquely downwards 

 and medially towards the centre of the organ, each traversing about one- 

 third of its total width, the remaining median third corresponds to a 

 zone where the anterior lobe is continuous with the substance of the 

 main posterior lobe. The structure of the two lobes is similar, but that 

 of the anterior lobe, especially its central part, is more homogenous. 

 Each lobe consists of a lobulated mass of small epithelial cells with deeply 

 stained oval nuclei. The most active growth is at the periphery, where 

 irregularly branched epithelial processes are growing out into the sur- 

 rounding vascular connective tissue. The epithelial cells appear to be 

 mostly directly derived from the inner ependymal zone of the diverticulum, 

 but there are also a certain number of cells which have the character of 

 the nuclear or mantle zone, which may be distinguished by their position 

 and by their nuclei being vesicular and pale in colour, as contrasted with 

 the deeply stained nuclei of the undifferentiated ependymal cells. At 

 the surface branched finger-shaped or club-shaped processes interdigitate 

 with vascular processes, which appear to grow inward between the 

 epithelial cords (Fig. 214, B, C, D). Further, if cross-sections of the 

 interdigitating processes are observed, it will be seen that the vascular 

 processes of pial tissue appear paler than the surrounding zones of densely 

 packed small epithelial cells. The vascular mesenchymal areas are at 

 first separated from contact with the neural epithelium by the external 

 limiting membrane, between which and the deeply stained ependymal 

 cells are a few sparsely scattered cells, with pale vesicular nuclei belonging 

 to the mantle zone. The general arrangement of the epithelial tubes or 

 cords and their relation to the ingrowing vascular processes, are seen 

 with diagrammatic clearness in Fig. 217, B and D, photographed from a 



