THE HUMAN PINEAL ORG AN— DEVELOPMENT 383 



producing either neuroglia cells or nerve cells even after the outward 

 migration of the indifferent cells from the internal limiting membrane 

 had already occurred. Whether an actual migration of whole cells takes 

 place or simply an outward movement of the nucleus along a protoplasmic 

 strand which has grown out as a process from the body of the cell is a 

 much debated question, but we are inclined to believe that the latter 

 alternative is the correct interpretation of the changed position of the 

 nuclei. The migration by amoeboid movements of microglial cells has, 

 however, been definitely proved and recorded on cinematograph films. 



It has been demonstrated by Cajal that at an early period of embryonic 

 life the spongioblasts send out a process which on reaching the external 

 limiting membrane expands into a conical swelling or foot-plate, while 

 the inner end, retaining its attachment to the internal limiting membrane, 

 develops one or more processes resembling cilia which project into the 

 central canal or a ventricle of the brain (Fig. 223, Chap. 22, p. 322, and 

 Fig. 261). The foot-plate at the outer end is in relation with the vessels 

 of the pia mater outside the external limiting membrane, and it is believed 

 that it serves to absorb material from the blood for the nourishment of 

 the nerve tissue, for in the earlier stages of development the latter does 

 not possess the rich blood supply which it has in late embryonic stages 

 and in foetal life. In these later stages, when the central area of the nerve 

 tissue has become vascularized, foot-plates are developed on those pro- 

 cesses of the neuroglia cells which come into relation with the sheaths 

 of ingrowing vessels (Fig. 220, p. 318), and it is said that with further 

 evolution there is a tendency for the sub-pial expansions to disappear and 

 eventually for the attachment of the inner end of the cell to the internal 

 limiting membrane to be lost. Branched lateral processes from the body 

 of the cell have in the meantime been developed, and eventually one or 

 more thick processes of the cell which have become attached by a foot-plate 

 to the sheath of a vessel constitute what are known as its vascular pro- 

 cesses, while the others, namely, the dendritic processes, either end freely 

 or communicate with similar processes of adjoining neuroglia cells. The 

 foot-plates on the vascular processes of the neuroglia cells are somewhat 

 like the club-shaped expansions of the parenchyma cells of the pineal 

 body (Fig. 218, Chap. 22, p. 317), and it is possible that serving as points 

 of attachment, the latter have the same function as the foot-plates, namely, 

 that of absorbing nourishment from the blood circulating in the vessel to 

 which the expansion is attached. 



In both neuroglial and pineal parenchyma cells the terminal expansions 

 are frequently conical or trumpet-shaped, but the expansions of the 

 neuroglial processes more often have the form of flat oval plates applied 

 to the surface of the vascular sheath (Fig. 262), whereas the ends of the 



