4 E. V. COWDKY 



pression between the superior colliculi, and is covered over with pia. It 

 is dark red in color and measures about six millimeters long and four 

 millimeters broad. The body is popularly considered to be larger in 

 women than in men, but careful studies on rats have failed to reveal any 

 sex differences (Hoskins(&) ). It is, however, relatively larger in chil- 

 dren than in adults. 



Embryology 



The pineal body first makes its appearance during the fourth or fifth 

 week of development as an outpouching of the roof of the diencephalon 

 just cephalad to the posterior commissure. It is formed first of ependyma 

 only, but the walls soon thicken and mesodermal elements become included. 

 It attains its highest development in children of about seven years of age, 

 after which involutionary changes begin to take place. 



Comparative Anatomy 



From the standpoint of comparative anatomy, the pineal body, like the 

 paraphysis, is simply a dorsal evagination of the neural tube. In some 

 lizards it becomes highly specialized and may perhaps function as a rudi- 

 mentary eye. It is never, at any stage in phylogeny, associated with 

 the alimentary tract, but invariably develops from ectoderm which pos- 

 sesses sensory rather than internal secretory potencies. 



Histology 



In the fully developed condition the pineal body consists of scattered 

 ependymal cells in a framework of neuroglia and connective tissue. The 

 ependymal cells tend to be arranged in lobules, which are partially marked 

 off by septa penetrating into the organ from the pial investment. Within 

 the lobules cyst-like structures may be recognized and a variable amount 

 of acervulus, or brain sand, is met with. The nerve supply is said to 

 come from the optic thalamus. The blood vessels invade the pineal body 

 with the connective tissue. They are large and sinusoidal. The ependy- 

 mal cells are of irregular shape. They contain a good deal of pigment, 

 as well as some lipoid inclusions and typical mitochondria. The reticular 

 or canalicular apparatus has not been investigated. Except when the cells 

 are arranged in the walls of cysts, there is no suggestion whatever of 

 polarity. ~No secretion antecedents have been found. A careful study 

 made with modern methods of technic is much needed. 



A complete review of the literature up to 1915 is given by Studnicka. 

 Some investigators are inclined to interpret the high vascularity, lobula- 



