AVUS. 



147 



ed as invaginations of the external epiblast ; unlike them they are 

 never closed up. 



During the second and third days there are formed the visceral or 

 branchial clefts, homologous with those of the Ichthyopsida, though 

 never developing branchial processes from their walls." 



They are however real clefts or slits passing right through the 

 walls of the throat, and are placed in series on either side across the 

 axis of the alimentary canal, lying not quite at right angles to that 

 axis nor parallel to each other, but converging somewhat to the 

 middle of the throat in front (fig. 112 and fig. 113) 



cv 



AOA 



Fig. 114. Section through the hind-bkain of a Chick at the end of the 

 third day of incubation. 



IV. Fourth ventricle. The section shews the very thin roof and thicker sides of 

 the ventricle. Gh. Notochord ; CV. Anterior cardinal vein ; CC. Involuted auditory 

 vesicle ; GC points to the end which will form the cochlear canal ; jRL. Becessus 

 labyrinth! (remains of passage connecting the vesicle with the exterior) ; lixj. Hypoblast 

 lining the alimentary canal ; /fO., AOA. Aorta, and aortic arch. 



Four in number on either side, the anterior is the first to be 

 formed, the other three following in succession. They originate as 

 pouches of the hypoblast, which meet the epiblast. At the junction 

 of the epiblast and hypoblast an absorption of the tissue is effected, 

 placing the pouches in communication with the exterior. 



No sooner has a cleft been formed than its anterior border {i.e. the 

 border nearer the head) becomes raised into a thick lip or fold, the 

 visceral or branchial fold. Each cleft has its own fold on its anterior 

 border, and in addition the posterior border of the fourth or last 

 visceral cleft is raised into a similar fold. There are thn^five visceral 

 folds to four visceral clefts (figs. 112 and 113). The last two folds how- 

 ever, and especially the last, are not nearly so thick and prominent as 

 the other three, the second being the broadest and most conspicuous 



10—2 



