VERTEBRATES FROM A CRUSTAOEAN-LIKB ANCESTOR. 387 



exists on the ventral side leading into the saccus vasculosus or 

 saccus infundibuli, the thin walls of which come to the surface 

 of the brain on the ventral side. The bag of the saccus vas- 

 culosus is limited in front by the optic chiasma, which separates 

 it from the recessus chiasmaticus ; its cavity is continuous 

 backwards into that of a well-defined tube which lies in the 

 middle line and extends nearly up to the large ventral fissure 

 which separates the prechordal and epichordal portions of the 

 brain. This tube is apparently called by W. Miiller (5) the 

 processus iufundibuli ; it is called by Ahlborn (3) the thick- 

 walled lobus infundibuli ; and is compared by him with the 

 unpaired lobus infundibuli of the Amphibia. It is figured 

 L. i. in his drawings, and is shown in transverse section in 

 figs. 27 and 28, PL xiv. The walls of this tube are composed 

 of ependyma continuous with that of the central cavities of the 

 brain, and as it approaches its termination its lumen becomes 

 occluded by the coming together of its walls. Near its ter- 

 mination it is in very many cases found to be lying outside the 

 brain, being separated from the nervous matter by lines of 

 pigment continuous with the pigmented tissue around the 

 brain. The appearance which it then presents is given in fig. 3, 

 PI. XXV, which is a section through a carmine preparation of 

 the brain of a specimen which had just undergone transforma- 

 tion. In fig. 2 I give another section from the same series a 

 few sections nearer the infundibulum, to show the commencing 

 separation of this tube from the brain-substance. In fig. 2 the 

 section has cut the last portion of the right ganglion habenulse, 

 and the notochord is not yet involved in the section ; it corre- 

 sponds very nearly with fig. 27, PI. xiv, of Ahlborn. In fig. 3 

 the notochord has just made its appearance, and the section 

 passes through the posterior commissure ; it corresponds so 

 closely with Ahlborn^s fig. 26 that the very next sections 

 contain the two large nerve-cells pictured by Ahlborn in that 

 figure. The comparison of his figure and mine shows that 

 he had really before him the closed end of this tube, biit 

 he has misinterpreted its meaning and labelled it 31. B., or 

 Meynert's bundle. 



