VI.] THE VISCERAL CLEFTS. 163 



mentary canal, the three layers of the blastoderm are 

 successively traversed, without any breach of continuity, 

 save such as is caused by the cavities of the blood- 

 vessels. In this neck, so constituted, there appear on 

 the third day certain fissures or clefts, the visceral or 

 branchial clefts. These are 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 and 

 j^arallel to each other, but converging somewhat to the 

 middle of the throat in front (Fig. 56). Viewed from 

 the outside in either fresh or preserved embryos they 

 are not very distinctly seen to be clefts ; but when they 

 are seen from within, after laying open the throat, their 

 characters as elongated oval slits can easily be recog- 

 nised. 



Four in number on either side, the most anterior is 

 the first to be formed, the other three following in suc- 

 cession. Their formation takes place from within out- 

 wards. The hypoblast is pushed outwards as a pouch, 

 which grows till it meets the epiblast, which is then 

 broken through, while the hypoblast forms a junction 

 with the epiblast at the outside of the throat. 



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 thus five 

 visceral folds to four visceral clefts (Fig. 56). The last 

 two folds however, and especially the last, are not nearly 

 so thick and prominent as the other three, the second 



112 



