342 JOHN WARREN 



point where the optic commissure develops. The diencephalon 

 in early stages shows signs of divisions into two segments. In 

 figs. 2 and 17, the differentiation is best marked in the roof. The 

 first segment is limited in front by the velum and the ridge con- 

 tinuous with the velum. In the floor appears the anlage of the 

 optic chiasma, the infundibular recess and the mammillary region. 

 Its caudal limit is the tuberculum posterius. The roof in these 

 early stages forms a low arch. From this will develop the post 

 velar arch, the diencephalic plexus, the supra commissure and the 

 epiphysis, the latter forming the caudal limit in the roof. This 

 segment is the parencephalon the first diencephalic neuromere or 

 segment. The second segment is narrower and somewhat wedge 

 shaped. Its roof forms a short arch ending behind at a groove 

 marking the cephalic limit of the mid brain. This part of the 

 roof becomes the pars intercalaris or synencephalon and in it 

 appears the posterior commissure. Later the hinder part of the 

 commissure extends backwards into the mid brain, but at first 

 it seems to be wholly confined to this segment, figs. 7 and 19. 

 Below, the second fore brain segment is narrow and its lower 

 boundary extends from the tuberculum posterius to the highest 

 part of the habenular arch, figs. 2, 3, and 4. In the turtle the seg- 

 ment at first has a cavity in its upper part only, which ends below 

 in the thickened ridge separating diencephalon from mesencepha- 

 lon, (see fig. 17) and is poorly marked at this early stage, but 

 later, however, figs. 18 and 19, it becomes broader both dorsally 

 and ventrally. In both lizard and turtle its floor lies between the 

 tuberculum posterius and the apex of the habenular flexure. Its 

 caudal and cephalic limits are formed by slight ridges, but its 

 cephalic limit on the inner aspect of the brain is not very 

 distinct. 



Seen from the external side, fig. 4, the ridge separating telen- 

 cephalon from diencephalon appears as a groove passing behind 

 the optic stalk. Behind this is a swelling best marked towards 

 the dorsal aspect of the brain which is the first diencephalic 

 segment or parencephalon, /. D. Behind this comes a smaller 

 swelling separated by a slight groove from the first. This is the 

 second diencephalic segment or synencephalon, //. D. Compare 



