18 FREDERICK TILNEY AND LUTHER F. WARREN 



pineal region develops a series of structures which seem to be 

 directly concerned with the formation of the fluid in the cavities 

 of the brain. He holds that the chorioid plexus supplies the 

 main bulk of this fluid, but the gland-like organization of the 

 paraphysis indicates that it may supply a secretion of special 

 chemical substances to the encephalic fluid. The organ reaches 

 its highest degree of development in amphibia, where it becomes 

 a large, complicated, glandular structure with a central canal 

 from which a complicated set of anastomosing tubules are given 

 off. It has a well-marked sinusoidal type of circulation. These 

 conditions have been observed by Warren 416 in Siredon, Nec- 

 turus, Proteus, Siren, Ichthyophis, Triton, Rana, Amblystoma, 

 and Diemyctylus. The paraphysis has a well-developed, 

 glandular character in amphibians and lizards; in birds it is 

 reduced to a single, thick-walled outgrowth of small dimensions. 

 Selenka 352 in 1890 observed the organ in opossum; it has also 

 been observed by Warren ('17) 417 in the sheep, and also by 

 Francotte 127 in 1887 in the human embryo. The paraphysis is 

 much reduced in the upper and lower ends of the vertebrate 

 series, while in the middle, especially in amphibia, it is much 

 developed. In amphibia its character is glandular, as it is also, 

 to a less degree, in reptiles. 



The paraphysis was erroneously regarded as the conarium by 

 Selenka ('90). 352 It has also been called the anterior epiphysis 

 by Burckhardt ('90) 42 and the pre-paraphysis by His ('68). 182 

 Sorensen ('94), 363 called it the posterior chorioid plexus. 



The velum transversum. This is a transverse furrow, imme- 

 diately caudad to the paraphysis, which projects into the ventricle 

 and separates the paraphysis from the dorsal sac. In some 

 instances this furrow is simple and flat, but in others it is thrown 

 into many subsidiary folds and becomes highly vascular in the 

 form of a plexus. In some forms, as in Peiromyzon, it is alto- 

 gether wanting, and under such circumstances the paraphysis 

 passes over without sharp line of demarcation directly into the 

 dorsal sac. In Chimaera there is a lack of the velum and also 

 -a small paraphysis so that the dorsal sac seems to pass over 

 into the lamina supraneuroporica without demarcation. In 



