148 



VISCERAL ARCHES. 



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 somato- 

 pleure 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 promi- 

 nence, 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. 113). This branch, starting from near the outer end of the 

 fold, runs forwards and upwards in front of the stomodseum, tending 

 to meet the corresponding branch from the fold on the other side, at 

 a point in the middle line nearer the front of the head than the 

 junction of the main folds (fig. 113 sm). The two branches do not 

 quite meet, being separated by a median process, which at the same 

 time grows down from the extreme front of the head, and against 



Me. 



Fig. 115. Transverse section through the dorsal region of an Embryo Chick oi' 



45 hours. 

 M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct ; So. Somato- 

 pleure; S.p. Splanchnopleure; p.p. pleuroperitoneal cavity; ao. aorta; v. blood- 

 ^essels; w. germinal wall; ch. notochord; op. junction between area opaca and area 

 pellucida. 



