THE PINEAL BODY 19 



Dipnoians the velum presents a pair of folds or it may develop, 

 as in certain amphibia, as an unpaired chorioid plexus. 



The dorsal sac. This element of the pineal region was called 

 the Zirbelpolster by Burckhardt 42 in 1890, the parencephalon 

 by Kupffer 222 in 1887, and the post-paraphysis by Sorensen 362 

 in 1893. Goronowitsch ('88) 153 appears to be the first to apply 

 to it the term dorsal sac. This sac is a dilated vesicle usually 

 extending far above the roof-plate. In mammalia, however, 

 in those forms in which the corpus callosum has made its appear- 

 ance, the sac becomes much flattened and is difficult to recognize 

 because of the altered condition consequent upon the develop- 

 ment of the corpus callosum. The walls of the dorsal sac are 

 lined internally by ependymal cells. In many instances these 

 walls may be thin and definite or quite thick, containing many 

 folds which may or may not be vascular; in certain instances 

 these folds attain such a vascularity that they resemble a chorioid 

 plexus. 



The pars intercalaris anterior. The more caudal portion of 

 the dorsal sac as it approaches the level of the roof-plate may 

 become much thickened and contain a dense mass of neuroglia 

 tissue. Usually this intercalated portion is not of any great 

 extent. It appears only in a few forms. 



The commissura habenularis. This element was called by 

 Osborn 288 in 1884 the superior commissure and by Gottsche in 

 1835 154 the commissura tenuissima. It affords a connection 

 between the two ganglia habenulae. In some cases, as in Petro- 

 myzon, the connection established by this commissure is such as 

 to include the mass of the two ganglia in the general commissural 

 region. In the immediate neighborhood of this commissure and 

 coming into direct connection with it is often seen the ending of 

 the nerve from the parapineal organ. This is particularly the 

 case in ^aurians, and it is by this means that the so-called parietal 

 nerve makes its connection with the brain. Its fibers may be 

 traced in some instances as far as the ganglia habenulae. 



The epiphyseal complex. This complex comprises two distinct 

 elements, a pineal and a parapineal organ. The pineal organ 

 may consist of an end-sac or terminal vesicle, a stalk, a proximal 



