254 H. E. JORDAN 



onto the surface. The lumen is spun across with a wide-meshed 

 network of very delicate fibrils (fig. 7) probably a coagulation 

 product. The cells surrounding the lumen are of the usual type, 

 irregularly columnar, non-ciUated and full of granules. The 

 structure appears to be due to the accumulation of a secretion 

 product which exerts pressure peripherally and compels the 

 arrangement of the adjacent cells in a manner to simulate an 

 alveolus. Structures appear at this stage which suggest the inter- 

 pretation of an invagination of such an alveolus by a pial trabec- 

 ula with capillaries, forming a sort of glomerulus (fig. 5). 

 Numerous transition stages appear between merely large inter- 

 cellular vacuoles and the alveoli or cysts. 



THE 17 CM. STAGE OF DEVELOPMENT 



Nothing essentially new appears at this stage. The pineal 

 body is considerably larger, the pial trabeculae are more numerous, 

 longer and coarser; antero-dorsally the lobulation is distinctly 

 marked on the surface and the amount of the melanic granules 

 has increased, especially in the superficial cells. Moreover, the 

 body is now ramified by connective tissue septa carrying delicate 

 blood-vessels, continuous with the pial trabeculae. These nu- 

 merous imperfect septa divide the parenchyma into folhcular 

 masses. Alveoli of varying calibre and length still remain. There 

 are also more numerous clearly defined parenchymal masses or 

 follicles, surrounding vascular trabeculae. No distinct ciliation 

 can be discerned on the cells Hning the alveoH. However the 

 cells lining the pineal recess are ciliated. 



THE 21 CM. STAGE OF DEVELOPMENT 



The pineal body of this stage of development (ca. 2^ months, 

 i.e., half of gestation period) has attained approximately four- 

 fold the bulk of the 15 cm. stage. It marks the period of the 

 greatest abundance of alveoli and melanic granules (fig. 6). The 

 latter are most abundant in the cells forming the walls of the 

 alveoli. The body appears distinctly black macroscopically at this 

 period. Later it has a dark grayish appearance. At full term its 



