382 THE PINEAL ORGAN 



are independent units. The discussion of this interesting and funda- 

 mentally important problem presents many difficulties, and we do not 

 propose to enter into the question here, since, apart from its general 

 bearing on the structure of nervous tissue, it does not specially affect 

 the study of the pineal organ ; for, whether the processes of the glial cells 

 are merely in contact or are continuous with each other, there seems to 

 be a functional continuity in the framework as far as support is concerned. 

 At this point it will be necessary to refer to the view which was 

 originally taken by His with respect to the origin of neuroglia cells and 

 neuroblasts. He believed that the medullary plate is primarily formed 



Fig. 261. — Transverse Section through the Spinal Cord of a Ten-day 

 Chick Embryo, showing the Supporting Cells. (After Lenhossek, 

 1895O 



a. : ependymal cell. c. : astroblast, or displaced epithelial 



b. : supportive spongioblast. cells. 



of undifferentiated cells which give rise to " germinal cells " and 

 " spongioblasts." The former in his opinion gave rise to neuroblasts 

 and ultimately to nerve cells, whereas the spongioblasts developed into 

 the supporting neuroglial tissue and ependyma. The germinal cells or 

 the cells next the internal limiting membrane which are undergoing 

 mitosis give rise to two daughter cells, both of which, he thought, migrated 

 outward through the spongioblasts into the mantle zone, where they 

 could be distinguished by their large size and pale vesicular nucleus, and 

 were spoken of as " neuroblasts." Schaper, in 1897, contended that the 

 germinal cells gave origin to both neuroblasts and spongioblasts, and that 

 the indifferent cells which resulted from their division were capable of 



