STRUCTURE OF FOUR-DAY CHICKS 



115 



(Figs. 42, A, and C, and 43). The telencephalic vesicles become 

 the cerebral hemispheres, and their cavities become the paired 

 lateral ventricles of the adult brain. The hemispheres undergo 

 enormous enlargement in their later development and extend 

 dorsally and posteriorly as well as anteriorly, eventually cover- 

 ing the entire diencephalon and mesencephalon under their 

 posterior lobes. 



metacoele 

 ( ventricle IV ) 

 thin roof of myelencephalon 



myelocoele 

 ( ventricle IV ) 



ventral cephalic fold 



spinal cord 



metencephalon 

 ganglion V 

 ganglion VII VIII 



meso-metenceohalic fold 



mesocoele 

 (Sylvian aqueduct) 



location of 

 posterior comm issure 

 meso-diencephalic fold 

 tuberculum posterius 

 diocoele( ventricle III ) 

 epiphysis 

 elum transversum 



ter*iSn"a1is / median telocoele 



{ ventricle III ) 



foramen of Monro 



lateral telencephalic 

 vesicle 



mesocoele 

 (sylvian aqueduct) 



metacoele 

 ^ ventricle IV ) 



myelocoele 

 ^ventricle IV) 



auditory vesicle 



Fig. 42. — Diagrams to show the topography of the brain of a four-day chick. 

 A, plan of sagittal section. The arbitrary boundaries between the various 

 brain vesicles (according to von Kupffer) are indicated by broken lines. B, 

 dextral view of a brain which has been dissected free. C, schematic frontal 

 section plan of brain. The flexures of the brain are supposed to have been 

 straightened before the section was cut. 



As a matter of convenience in dealing with the morphology 

 of the brain, more or less arbitrary Hnes of division between 

 the adjacent brain regions are recognized. The division be- 

 tween telencephalon and diencephalon is an imaginary line 

 drawn from the velum transversum to the recessus opticus 



