THE HUMAN PINEAL ORGAN — DEVELOPMENT 387 



membrane of one type of pineal cyst, and it seems possible that even 

 after the full development of the organ has been attained and all traces 

 of the original cavity have disappeared, certain cells which resemble 

 the embryonic spongioblasts or primary ependymal cells retain the power 

 of differentiating into either ependyma or neuroglia. This supposition 

 may also apply to the development of ependymal cells lining the cavities 

 in certain cases of syringomyelia, but when epithelium is found lining 

 cysts in the pineal body or the cavities in the grey matter of the spinal 

 cord it is usually of an irregular type — pseudo-ependymal — and the 

 typical ependyma of the tall columnar form is seldom seen. The variations 

 which occur in the normal ependyma and choroidal epithelium of the 

 ventricles and central canal of the spinal cord are very considerable. 

 These changes in the structure and form of the epithelium are related 

 to varying mechanical and other conditions which are present in different 

 parts of the brain and spinal cord. Thus the lining epithelium may be 

 modified in one situation to form a sensory epithelium, e.g. the retina 

 of the lateral eyes, and in another to form a secretory organ, as in the 

 choroidal epithelium of the ventricles ; and since ependyma enters 

 largely into the composition of the pineal organ as a whole, we propose 

 to give a short summary of the morphology and functions of the ependyma 

 in general, with the view of gaining a better insight into the structure 

 and possible functions of this tissue as it occurs in the epiphysis of 

 mammals. The morphology of the ependyma has been specially studied 

 by Agduhr and Studnicka, and the following brief note is largely based 

 on Agduhr's account of the ependyma in Penfield's Cytology of the 

 Nervous System. 



The ependyma is seen in its simplest form in the central nervous 

 system of Amphioxus and of cyclostomes, in which the supporting tissue 

 of the central nervous system is said to be wholly epithelial throughout 

 life. The term " ependyma," as usually understood, is applied to the 

 epithelial lining of the ventricular cavities of the brain and central canal 

 of the spinal cord, but in the embryonic condition in all vertebrates, before 

 the development of nerve cells and nerve-fibres, it extends through the 

 whole thickness of the wall of the neural tube and takes part in the forma- 

 tion of the internal and external limiting membranes. Later, when 

 the differentiation of this wall into zones has taken place, the term 

 " ependyma " is often applied to the inner zone, which consists of several 

 layers of spongioblasts, or " primary ependymal cells." This inner zone 

 at a later stage is seen to be further differentiated into the definitive 

 ependyma, glioblasts or astroblasts, and neuroblasts. Still later, the 

 supporting function of the neuroglial tissue is supplemented by the 

 ingrowth of vessels with their connective tissue sheaths. 



