120 THE THIED DAY. [CHAP. 



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 mesoblast of the body. 



Of the history of these most important visceral folds and 

 clefts we shall speak in detail hereafter ; meanwhile we 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. This branch, starting from 

 near the outer beginning of the fold, runs forwards and 

 upwards, 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. 

 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 which they 

 abut. Between the main folds, which are directed somewhat 

 downwards and the branches which slant upwards, a some- 

 what lozenge-shaped space is developed which, as the folds 

 become more and more prominent, grows deeper and deeper. 

 The main folds are the rudiments of the inferior inaxilke, 

 the branches those of the superior maxillce, the lozenge- 

 shaped cavity between them is the cavity of the mouth, 

 and the descending process which helps to complete the 

 upper margin of this cavity is called, from the parts which 

 will be formed out of it, the fronto-nasal process. 



Already on the second day the under wall of the front 

 end of the alimentary canal began to become thin, while over 

 it the epiblast was pushed in so as to form a depression. The 

 maxillary folds convert this depression into a deep pit, 

 whose bottom is not as yet perforated, the opening into 

 the alimentary canal being made later on. 



The two succeeding pairs of visceral folds are transformed into parts winch 

 will be best considered in connection with the development of the skull. The 



