360 JOHN WARREN 



In the turtle this plexus is much more extensive. It appears 

 at a relatively early stage, 16 mm., fig. 21, and at 26 mm., fig. 

 22, is highly developed. Here we have a distinct bilateral arrange- 

 ment as is shown in figs. 25 and 26. This is a striking feature of 

 the turtle's plexus and the rudiments of the telencephalic plexus 

 are also paired as was mentioned above. All the post velar arch 

 and dorsal sack from the opening of the epiphysis up to the 

 opening of the paraphysis is taken up in this plexus formation, figs. 

 22 and 23, and there is a sort of median ridge thrust in between 

 the paired masses. This plexus is well marked in serpents, 

 Leydig, fig. 5, and in Chelone imbricata, Voeltzkow (96), figs. 

 21-22. Herrick (38) shows a sagittal drawing from Sorensen 

 of the brain of Cistudo where the diencephalic as well as the 

 telencephalic plexuses are well developed. In Chrysemys I 

 could find no trace of the latter plexus. In birds the plexus of 

 the chick is described by Dexter (20), figs. 2-5, and by Kupffer 

 (57), fig. 289. The diencephalic plexus is well marked and in- 

 volves apparently the cephalic portion of the post velar arch only, 

 the caudal part remaining smooth. It completely covers up the 

 velum, and approaches more the condition seen in mammals. 



In mammals the diencephalic plexus is strongly developed, form- 

 ing large masses of plexus in the roof of the third ventricle which 

 extend from the velum back to the epiphysis. 



Commissures 



In the velum of the lizard of 37 mm., fig. 12, and in the adult, 

 fig. 13, is seen a well marked commissural band. This passes 

 from the median wall of one hemisphere through the lowest part 

 of the velum just caudal to the opening of the paraphysis to 

 reach the median wall of the opposite hemisphere. I have also 

 seen it in an earlier stage between figs. 11 and 12. This com- 

 missure was first mentioned by Rabl-Rlickhard ('81) in Psam- 

 mosaurus and called by him Tornix-rudiment,' Meji^er ('93) 

 named it the 'Hintere Mantel-kommissur,' Kupffer and Edinger 

 have called it Commissura pallii posterior. 



