VI,] THE NASAL PITS. 145 



tion, consists of somewhat dense mesoblast covered by epiblast 

 which on the summit is thickened into a sort of cap. The 

 front limbs or wings (Fig. 46) arise just behind the level of 

 the heart, and the hind limbs in the immediate vicinity 

 of the tail. The first traces of them can be seen towards 

 the end of the third, but they do not become conspicuous 

 till the fourth day, by the end of which the two pairs may 

 be already distinguished by their different shapes. The front 

 limbs are the narrowest and longest, the hind ones being 

 comparatively short and broad. Both are flattened from 

 above downwards and become more so as their growth 

 continues. 



6. In the head, the vesicles of the cerebral hemispheres 

 are rapidly increasing in size, overlapping the insignificant 

 olfactory vesicles in front, and encroaching on the 'tween- 

 brain or vesicle of the third ventricle behind. The mid-brain 

 is now, relatively to the other parts of the brain, larger than 

 at any pther epoch, and an indistinct median furrow on its 

 upper surface indicates its division into two lateral halves. 

 The great increase of the mesoblastic contents of the second- 

 ary optic vesicle or involuted retinal cup causes the two eye- 

 balls to project largely from the sides of the head (Fig. 48, Op). 

 The mass of mesoblast which invests all the various parts of the 

 brain, is not only growing rapidly below and at the sides, but 

 is also undergoing developments which result in the forma- 

 tion of the primitive skull, and of which we shall speak in 

 detail in a subsequent separate chapter. 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 arid more easy to recognize as that of a head. 



7. Meanwhile the face is also being changed. The two 

 nasal pits were on the third day shallow depressions with 

 thickened borders complete all round. As the pits deepen on 

 the fourth day by the growth upwards of their rims, a break 

 is observed in each rirn in the form of a groove (Fig. 48, N) 

 directed obliquely downwards towards the cavity of the mouth. 

 The fronto-nasal process or median ridge (Fig. 48, 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 fourth day becomes 

 increasingly prominent, separates the two grooves from each 



