202 THE FOURTH DAY. [CHAP. 



of the primitive skull. All these events, added to the 

 cranial flexure spoken of above, give to the anterior 

 extremity of the embryo a shape which it becomes more 

 and more easy to recognize as that of a head. 



Meanwhile the face is also being changed. The two 

 nasal pits were on the third day shallow depressions com- 

 plete all round. As the pits deepen on the fourth day 

 by the growth upwards of a rim round them, a deficiency 

 or break in the ridge may be observed on that side of it 

 turned towards the mouth; which constitutes a kind of 

 shallow groove (Fig. 69 N) directed obliquely downwards 

 towards the cavity of the mouth. The fronto-nasal 

 process or median ridge (Fig. 69, nf), which on the third 

 day rose up between the superficial projections caused by 

 the bulging anterior extremities of the vesicles of the 

 cerebral hemispheres, and on the foxirth day becomes 

 increasingly prominent, separates the two grooves from 

 each other, and helps to form the inner wall of each of 

 them, while the depth of the groove also becomes in- 

 creased by the prolongation along its inner side of the 

 rim surrounding the nasal pit. Abutting on the outer 

 side of each groove near the mouth and so helping to 

 form the outer wall of each, lie the ends of the superior 

 maxillary processes of the first visceral arch (Fig. 69 B, 

 SM), which like the fronto-nasal process are increasing 

 in size. By their continued growth, the groove is more 

 and more deepened, and leading as it does from the 

 nasal pit to the cavity of the mouth, may already be 

 recognized as the rudiment of the passage of the pos- 

 terior nares. 



During the latter half of the fourth day there ap- 

 pears at the bottom of the deep lozenge-shaped cavity 



