96 On the Morphology of the Pineal Region 



paraphysal gland very clearly distinguishable by, 1, the character of its 

 epithelium, 2, its tubular structure and 3, its apparently sinusoidal 

 circulation, from the choroid plexus proper, in the midst of which lies 

 the orifice of the gland. In reptiles the velum has been identified and 

 its morphological importance for the class emphasized by Burckhardt 

 (Anat. Anzeiger, IX, 320). In birds, Fig. li, the velum, 7, is almost 

 rudimentary in its median portion, though very broad, and it merges 

 without recognizable boundary into the very broad post-velar arch, 

 which can already be identified as the anlage of the tela choroidea. As 

 regards mammals further investigation is necessary, both to determine 

 the history of the velum and of the paraphysis, if that organ is present 

 in the placental, as Selenka states it to be in marsupial, mammals. In 

 both birds and mammals the lateral portions of velum, i. e. the chor- 

 oid plexus of the lateral ventricles is highly developed. It thus ap- 

 pears that as we ascend the vertebrate series there is first a broaden- 

 ing of the velum, and an increase of its lateral development, then occurs 

 a further reduction and flattening out of the velum, and a much 

 greater growth of the lateral plexus. 



The superior commissure is a remarkably constant structure in the 

 vertebrate brain, and must, since it persists in all vertebrate types, be 

 regarded as a fiber tract of fundamental morphological importance. 

 H. F. Osborn, 84.1, 268, was the- first to demonstrate the homologies, 

 wide occurrence and topographical relations of the tract, and to apply 

 the name " supra commissura." So far as I am aware, it has not yet 

 been recorded in birds, but a more thorough search will perhaps lead 

 to its discovery in that class. It has been found in representative types 

 of all other vertebrate classes, including mammals, for I have observed 

 it in embryos of rabbits, pigs and cats. In all cases it develops later 

 than the posterior commissure — in mammals much later, and is at first 

 ' much smaller than the posterior commissure. It acquires a large size 

 in acanthias and perhaps other fishes, it attains less size in amphibians 

 and is proportionately smallest in mammals. Its position is very con- 

 stant as it is always situated immediately in front of the orifice of the 

 epiphysis and at the outer surface of the brain wall. 



The posterior commissure belongs morphologically to the mid-brain, 

 not to the fore-brain. For the present the epiphysis may be accepted 

 as marking the posterior limit of the fore-brain. 



The pineal region develops a series of structures, which, from their 

 anatomical characteristics, appear to be directly concerned in the forma- 

 tion of the fluid in the cavities of the brain. We may assume that the 

 choroid plexus supplies the main bulk of the fluid, but the gland-like 



