428 THE FORE-BRAIN. 



The cavity of the mid-brain, greatly reduced in size in the 

 higher forms, is known as the iter a tertio ad quartum ventricu- 

 lum, or aqueductus Sylvii. 



The roof of the mid-brain is sharply constricted off from the 

 divisions of the brain in front of and behind it, but these 

 constrictions do not extend to the floor. 



In some Vertebrates the region of the mid-brain is stated to 

 undergo hardly any further development. In the Axolotl it 

 remains according to Stieda 1 as a simple tube with nearly uni- 

 formly thick walls. In the majority of forms it undergoes, how- 

 ever, a more complicated development. 



In Elasmobranchs the sides become thickened to form the optic lobes, 

 which are soon separated by a median longitudinal groove. The floor 

 becomes thickened to form the crura cerebri. The primitive simple median 

 cavity becomes imperfectly divided into a median portion below, and two 

 lateral diverticula in the optic lobes. 



In Teleostei the changes, resulting in the formation of (i) a pair of 

 longitudinal ridges projecting from the roof into the cavity of the iter, 

 constituting the fornix of Gottsche, and (2) of the two swellings on the floor, 

 forming the tori semicirculares, are more complicated, but have not been 

 satisfactorily worked out. In Bombinator and the Anura generally the 

 changes are of the same nature as those in Elasmobranchii, except that the 

 prolongations of the ventricle into the optic lobes are still further constricted 

 off from the median portion, which forms the true iter. 



In Reptilia and Aves the development of the mid-brain takes place on 

 the same type as in Elasmobranchii and the Anura. In Birds the optic 

 lobes are pushed very much aside, and the roof of the iter is greatly thinned 

 out. In Mammalia the sides of the mid-brain give rise to two pairs of 

 prominences the corpora quadrigemina instead of the two optic lobes of 

 other Vertebrata. The prominences, which do not contain prolongations of 

 the iter, become first visible on the appearance of an oblique transverse furrow, 

 while the anterior pair alone are separated by a longitudinal furrow. In the 

 later stages of development the longitudinal furrow is continued so as to 

 bisect the posterior pair. 



The floor, which is bounded posteriorly by the pons Varolii, becomes the 

 crura cerebri. The corpora geniculata interna also belong to this division 

 of the brain. 



Fore-brain. In its earliest condition the fore-brain forms a 

 single vesicle without a trace of separate divisions, but very 

 early it buds off the optic vesicles, whose history is described 

 with that of the eye. 



1 " Ueb. d. Bau d. centralen Nervensystem d. Axolotl." Zeit.f. wiss. Zool., Vol. 

 xxv. 1875. 



