328 JOHN WARREN 



corresponding degree of development, namely from 3 to 4.5 mm. 

 The post velar arch now forms a well arched roof to the dien- 

 cephalon ending at the epiphysis which is a short oblong body 

 opening by a very short hollow stalk into the diencephalon. It 

 extends forward and lies directly on the top of the post velar arch. 

 There is no sign now or at any later stage of any differentation 

 of this body into a pineal eye and an epiphysis. Behind it 

 is seen the last segment of the diencephalon, the intercalated 

 arch, S, which has been invaded by the first traces of the 

 posterior commissure, P.C., which is confined at this stage wholly 

 to the diencephalon as was the case in the corresponding stage 

 of Lacerta muralis, 4.5 mm., fig. 8. This stage corresponds to a 

 combination of the stages of Lacerta agilis of 3.6 mm., fig. 6, 

 and Lacerta muralis, 4.5 mm., fig. 8. 



Fig. 20 shows the brain of an embryo of 9.5 mm. The paraphy- 

 sis has now become a fairly long, round straight tube extending 

 vertically upwards parallel to the anterior wall of the post velar 

 arch. The close relations of the veins to it are clearly shown. 

 The post velar arch has developed very rapidly forming a high 

 vaulted roof to the diencephalon. Its walls, as was the case in 

 lacerta, are very thin and are covered over by a very complicated 

 net-work of small vessels, which will form later the diencephalic 

 choroid plexus. The epiphysis has elongated and is much the 

 same shape as the paraphysis, though smaller. It no longer opens 

 into the diencephalon but is attached to it by a solid stalk and is 

 closely surrounded by the venous plexus, Ve., overlying the post 

 velar arch. The blood from this plexus, as shown in fig. 19 is 

 carried off by several large veins passing downward on the lateral 

 aspect of the diencephalon which empty into the primitive 

 jugular vein. A similar arrangement is seen in Lacerta muralis 

 of 28 mm. and also in earlier stages. On either side of the stalk 

 of the epiphysis are seen the superior and posterior commissures. 

 The former appears first at this stage and in fact can only just 

 be seen in the specimen. It has been slightly exaggerated in the 

 model. The posterior commissure is increased in size, and overlaps 

 now slightly onto the wall of the mid brain. Its posterior limit 

 is here quite clear and definitely placed, but later, as was the case 



