356 JOHN WARREN 



In birds the velum has been described by Burckhardt (13), fig. 

 3, crow, and by Dexter (20) in the chick, fig. 1, where it forms a 

 distinct angle with a well marked post velar arch behind it. See 

 also KiipfTer (57) fig. 278 for a similar stage, and Burckhardt (13) 

 crow embryo. The post velar arch forms a high tent-like struc- 

 ture the cephalic wall of which becomes convoluted to form the 

 diencephalic plexus. The two walls are finally quite closely 

 approximated and the resulting condition resembles somewhat 

 that in the lizard. The plexus formation extends to the velum, 

 which is again reduced to a mere lip, and becomes so extensive 

 that the velum is practically covered up by the folds of the plexus, 

 Dexter (20), fig. 3, and Kupffer (57), fig. 289. In mammals John- 

 son (53) shows the velum in a series of pig embryos from 5-17 mm., 

 figs. 34-39. It forms at first an angle in the roof continued around 

 the lateral walls as a distinct fold, but steadily becomes smaller; 

 when at 17 mm. it is a mere notch in the roof. Behind it there 

 is a well marked post velar arch. 



Neumayer (75) gives a series of models of the brain of sheep 

 embryos from 2.5-16 mm. There is a well marked velum with a 

 low paraphysal arch behind it in embryos of 10 mm. In the 

 sheep, H. E. C. as 1334, 6.6 mm., the velum is distinct with a 

 well developed post velar arch. Later it greatly diminishes in 

 size and at 26 mm., H. E. C. no. 1112, fig. 38, becomes covered up 

 by the diencephalic plexus which develops from it and a large 

 part of the post velar arch. 



Ziehen (100), fig. 47, shows a sagittal section of a rabbit taken 

 from Neumayer. Here there is a slight fold in the fore brain 

 which Neumayer called the velum, but Ziehen has doubts whether 

 the velum of lower forms appears in mammals, p. 279. He 

 gives the boundary between diencephalon and telencephalon as 

 the fossa praediencephalica, but is uncertain whether this should 

 be homologized with the velum. His fig. 23, hedgehog embryo 

 after Groenburg, seems to show a distinct velum with a well 

 marked post velar arch behind and there is a distinct diencephalic 

 plexus in the arch. 



W. His (47), figs. 33, 35, and 38, shows a distinct velum in 

 models of human embryos. In the lower forms, except in Petro- 



