V.] THE VISCERAL CLEFTS. 119 



observed between it and the actual head. In other words, 

 a distinct neck has been formed, in which most important 

 changes take place. 



The neck is distinguished from the trunk in which the 

 heart now lies by the important feature that in it there is no 

 cleavage of the mesoblast into somatopleure and splanch- 

 nopleure, and consequently no pleuroperitoneal cavity. In 

 passing from the exterior into the alimentary 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 ap- 

 pear 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 parallel to each other, 

 but converging somewhat to the middle of the throat in front 

 (Fig. 37). Viewed from the outside in either fresh or pre- 

 served 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 

 recognised. 



Four in number on either side, the most anterior is the 

 first to be formed, the other three following in succession. 

 Their formation takes place from within outwards. The 

 hypoblast and mesoblast are first absorbed along the line 

 of the future cleft, then the epiblast is broken through, and 

 the hypoblast, which is carried outwards as a lining to the 

 slit, forms a junction with the epiblast at the outside of the 

 throat. 



No sooner has a cleft been formed than its upper border 

 (*. 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 upper border, and in addition the lower 

 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. 37). The last two folds however , and 

 especially the last, are not nearly so thick and prominent 

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

 conspicuous of all. The first fold meets, or nearly meets its 

 fellow in the middle line in front, but the second falls short 



