92 Journal of Comparative Neurology. 



ignate it. If the writer understands His correctly it corres- 

 ponds to the Rautenhppe. See kl., Fig. 28, PI. Ill, 



The appearance of the early epencephal is well shown in 

 Figs. I, 3, 4, 5, 6, PI. I, and in Figs. 50, 51, 55, PI. V. In 

 Figs. 1-3, PI. I, the pons flexure is just beginning to appear. 

 Its further development causes a greater separation of the meso- 

 caudal border of the epencephal till it comes to lie in the posi- 

 tion shown in Figs. 4, 5, 6 and 7, PI. I. These specimens also 

 show the dorso-cephalic rotation of the two lateral protons of 

 the cerebellum; compare also Figs. 31, 35, 37, PI. Ill, and 

 Figs. 50, 51, 55, PI. V. 



In order to understand the changes which follow we must 

 consider those which have preceded. 



1. The brain tube was once an open groove. 



2. The lateral walls of this groove have extended dorso- 

 mesad and fused along the meson. Now we would naturally 

 expect that the sides of the tube would be the thicker and con- 

 tain potential proliferating elements. Does not this idea ex- 

 plain why the Deckplatte is so thin ? 



As the pons flexure is developed, the kilos and a part of 

 the substantial wall are forced ectad and in this manner the lat- 

 eral U-bend is formed. The ectal opening between the two 

 arms of the U is closed by a portion of the original metatela 

 which is thus folded off, and later becomes the dorso-caudal wall 

 of the parepicoele. See Figs, i, 3, 5, 12, PI. I, Fig. 60, PI. 

 VI, and Fig. 65, PI. VII. 



Dorsad of the pons flexure, a fold of the metatela sinks 

 down into the cavity, bloodvessels follow and form a plexus 

 (choroid plexus of the older writers) which extends clear across 

 the brain tube. The place where the fold occurs forms the line 

 of attachment for the plexus ; it encircles the dorsal part of the 

 brain tube like a girdle, hence the name cestus is proposed for it. 



From the foregoing it appears that that portion of the 

 brain tube which lies between the isthmus and the pons flexure 

 presents peculiar characters, such as are not presented by the 

 parts lying next cephalad or caudad of it. 



The question which now demands attention is, How much 



