150 FREDERICK TILNEY AND LUTHER F. WARREN 



It is situated close to the roof of the interbrain, its long axis 

 being as a rule, perpendicular to the latter. In no instance does 

 it approach or come in contact with the inner surface of the 

 skull. 



The histology of the structure discloses several different 

 forms which the organ may assume. Studnicka ('05), 391 distin- 

 guishes three distinct types: 1) A long sac with thick walls con- 

 taining many follicles. Such an organ is found in Passer Gage 

 ('95). 136 2) A solid body with communicating or independent 

 acini which connect with the lumen of the pineal body by means 

 of a still potent canal. Between the follicles are many blood 

 vessels and much connective tissue. The stalk is solid as in 

 Meleagris gallopavo Mihalkovicz ('77). 275 3) A solid organ in 

 which there are solid, blind acini instead of hollow follicles. 

 These acini make up solid lobules. 



In brief, these three types may be termed, 1) saccular; 2) 

 follicular or acinal, and, 3) solid. There are a number of 

 transitional forms in addition to those already mentioned. 



Funkquist 133 in 1912 describes two morphogenetic types in 

 birds. 



1. The organ has a simple tubular character which, during 

 growth, shows a thickening of its walls and a general enlarge- 

 ment. In some cases the organ is solid except at its base where 

 it retains a cavity, the recessus pinealis. 



2. In this type the organ has a tubular character, in many 

 instances retaining its connection with the original pineal evagi- 

 nation and in others being cut off from it. These bud-like 

 tubular processes resemble tubuli of the dorsal sac. The pineal 

 organ has its original anlage in an epithelial structure. Later, 

 development causes a transition into neuroglia tissue in much 

 the same way as the transition occurs in the central nervous 

 system. In some cases (canary and turkey) the acinus-for- 

 mation, giving rise to simple pineal tubules, persists, while in 

 other instances these acini are more or less obliterated. 



Two types of cells may be identified, according to Funkquist, 

 namely, large epithelial cells with clear protoplasm and small 

 darkly staining cells. 



