THE PINEAL BODY OF THE SHEEP 253 



with a thickening of the wall and obliteration of the lumen in the 

 terminal portion. The cells of this solid portion are mainly of 

 the irregularly polygonal and stellate type and form a loose- 

 meshed syncytial network with delicate fibrils in all respects 

 similar to that of the 5 cm. stage. Considerably more of the cells 

 contain the melanic granules and a number of all types are in 

 mitosis. The vesicular nuclei contain numerous karyosomes. 

 This stage marks also the beginning of the pial extensions into 

 the substance of the body forming trabeculae (ca. two per section) 

 carrjdng delicate blood-vessels (fig. 2). Fig. 3 illustrates transi- 

 tion stages between ependyma and neuroglia cells. 



THE 11 CM. STAGE OF DEVELOPMENT 



At this stage the pineal body has acquired the shape of a thick, 

 rounded, cellular knob with shallow central recess. The walls of 

 the latter contain anteriorly the now well-developed habenular 

 or superior commissure. The parenchyma is more loosely ar- 

 ranged peripherally. The cells are of the same types as above 

 described, more and less developed neuroglia cells. Slightl}' 

 more contain melanic granules, and in somewhat larger amounts. 

 Fig. 4 h illustrates the various types of these granules. Many 

 cells are dividing mitotically. No amitosis can be detected. 

 More pial trabeculae enter the body (ca. three per median sec- 

 tion), dividing it peripherally into lobules (four per section). 

 Fig. 4 a illustrates the early stage in the differentiation of the 

 inter-neuroglia cells. 



THE 15 CM. STAGE OF DEVELOPMENT 



This stage is characterized by shghtly larger size, and consid- 

 erably larger amount of melanic cytoplasmic granules, more abun- 

 dant in the peripheral cells; but more particularly by the appear- 

 ance of a new feature ; several short blind alveoli or cysts situated 

 in the region approximately midway between periphery and center. 

 From four to six alveoli appear in a section. They extend 

 through from eight to ten sections of ten microns, and seem not to 

 branch. Nowhere can they be traced either into the recess or 



