178 VISCERAL ARCHES. 



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)- 



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 thus Jive visceral folds to four visceral clefts (figs. 112 

 and 1 1 3). 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 of reaching the middle line, and 

 the third, fourth and fifth do so in an increasing degree. Thus 

 in front views of the neck a triangular space with its apex 

 directed towards the head is observed between the ends of the 

 several folds (fig. 113 A). 



Into this space the pleuroperitoneal cavity extends, the 

 somatopleure separating from the splanchnopleure along the 

 ends of the folds ; and it is here that the aorta plunges into the 

 mesoblast of the body. 



The history of these most important visceral folds and clefts 

 will be dealt with in detail hereafter ; meanwhile I may say that 

 in the Chick and higher Vertebrates the first three pairs of folds 

 are those which call for most notice. 



The first fold on either side, increasing rapidly in size and 

 prominence, does not, like the others, remain single, but sends 

 off in the course of the third day a branch or bud-like process 

 from its upper edge (fig. 1 1 3). This branch, starting from near 

 the outer end of the fold, runs forwards and upwards in front 

 of the stomoda^um, tending to meet the corresponding branch 



