THE LATER DEVELOPMENT OF THE CHICK 307 



thickened as the optic thalami. The roof remains thin and 

 considerably expanded; in the region of the .velum transversum 

 the roof of the diencephalon and telencephalon becomes modi- 

 fied into the folded choroid plexus of the third ventricle (Fig. 124). 

 Back of this the diencephalic roof is evaginated as the tubular 

 epiphysis or pineal body. Finally a thickening in the dorsal 

 wall marks the posterior dorsal limit of the diencephalon; this 

 is the rudiment of the posterior commissure. 



The mesencephalon, or mid-brain, forms the topographically 

 anterior part of the brain, projecting considerably in advance 

 of all the other structures of the embyro (Figs. 113, 114, 123). 

 It includes the fourth and fifth neuromeres. It remains 

 relatively undifferentiated for a considerable period, but later 

 its dorsal and dorso-lateral walls evaginate and thicken, form- 

 ing the optic lobes, which include a part of the original cavity 

 of the mesencephalon. The central portion of its cavity, now 

 greatly restricted, remains as the aqueduct of Sylvius, or iter, 

 continuing posteriorly from the third ventricle. The ventral 

 and ventro-lateral walls of the mesencephalon thicken to form 

 the great nervous pathways from the optic lobes to other cen- 

 ters, known as the crura cerebri. Back of the optic lobes the 

 mesencephalon remains undilated as the isthmus, leading 

 directly to the metencephalon (Fig. 124). 



The metencephalon, lying dorsal to the anterior tip of the 

 notochord, consists of only the sixth neuromere, and is thus 

 very short. The cavity of the metencephalon is not separable 

 from that of the succeeding section, the myelencephalon; to- 

 gether they are known as the fourth ventricle. The walls of the 

 metencephalon thicken slowly, though steadily, during the 

 early days of incubation. Later the dorsal and dorso-lateral 

 regions become very greatly thickened, forming the cerebellar 

 hemispheres, or cerebellum. The ventral and ventro-lateral 

 walls also thicken very considerably to form the pons Varolii. 



The myelencephalon, the last division of the brain, includes 

 the seventh to eleventh neuromeres, indications of which re- 

 main visible, ventrally, until about the fourth day. Its cavity, 

 as said above, is the fourth ventricle. The roof of this section 



