Johnston, Forcbrain Vesicle in Vertebi-ates. 459 



This forms the neural part of the hypophysis in mammals and pro- 

 jects more or less directly downward from the tuber cinereum. The 

 connection of the sac with the tuber cinereum is the infundibulum 

 and its cavity may be called the infundibular cavity or recess. The 

 posterior part of the ventral wall forms a second bilobed structure, 

 the corpora mammillaria. 



The telencephalon shows greater changes of form and size in 

 different vertebrates than any other segment of the brain. At the 

 same time the essential morphological relations are completely pre- 

 served throughout the series from cyclostomes to man. Tt is part 

 of the purpose of this paper to make this fact more clear and ex- 

 plicit ^\\i\\ regard to certain features of the forebrain, but those 

 features upon which there are no differences of opinion may first be 

 sketched. The telencephalon is very deeply bilobed — bifurcated — in 

 front. Each forward prolongation receives an olfactory nerve and 

 is known as the olfactory bulb. This may be closely applied to the 

 body of the telencephalon as in cyclostomes and amphibians, or it may 

 be removed by a longer or shorter distance as in most other classes. In 

 the latter case there is an obvious olfactory tract. The body of the 

 telencephalon is always bilobed, the lateral halves being joined by 

 thinner portions which are usually membraneous or thickened only 

 by nerve fibers. Only in selachians is there a considerable thicken- 

 ing of the median part of the roof by gray matter. The ventricle 

 which continues forward from that of the diencephalon is always 

 bifurcated, each lateral division extending into the olfactory bulb 

 (except in some adult mammals where it becomes obliterated during 

 development). A membraneous roof continuous with the tela cho- 

 rioidea of the third ventricle extends over the whole length of 

 the telencephalon except the olfactory tracts and bulbs. In all 

 classes the lateral halves of the telencephalon are connected at the 

 rostral end by the lamina terminalis, a membrane formed by the 

 closing of the anterior neuropore of the embryo. This lamina is 

 always thickened by transverse fibers of two or more systems and 

 sometimes divided into two bundles, the commissura anterior. At 

 the lower border of the lamina terminalis is a prominent depression 

 which lies just in front of the optic chiasma and is therefore called 



