164 THE THIRD DAY. [CHAP. 



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. 



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 mesobkst of the body. 



The visceral clefts and arches to a large extent dis- 

 appear in the adult, and constitute examples of an inte- 

 resting class of embryonic organs, whose presence is 

 only to be explained by the fact that, in the ancestors of 

 the types in which they are now developed in the 

 embryo, they performed an important function in the 

 adult. The visceral arches and clefts are in fact the 

 homologues of the branchial arches and branchial clefts 

 of Fishes, which continue to be formed in the embryos 

 of the higher vertebrate types, although they have 

 ceased to serve as organs of respiration. The skeletal 

 structures developed in the visceral arches persist as 

 the jaw-bones and hyoid bone, but the clefts, with the 

 exception of the first, become obliterated. 



Of the history of the skeletal elements we shall 

 speak in detail hereafter; meanwhile we may briefly 

 deal with the general history of these parts. 



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 anterior edge. This 



