290 



ANIMAL PHYSIOLOGY. 



separated by clefts ; and the foremost of these, uniting with its 

 fellow in the middle line, forms the lower jaw, while the 

 upper part of the cleft behind it remains as the opening of the 

 ear, and the other clefts entirely disappear in the higher verte- 

 brates, although they are undoubtedly the same clefts as those 

 which in fishes separate the gills throughout life. In front 

 of the first branchial process, the eye is developed in a cleft 

 of the cephalic plate, and separated by a little process (lateral 

 frontal) from the nostril. Between the nostrils, the mesial 

 termination of the cephalic plate is prolonged down to 

 form the middle portion of the upper lip, and is called the 

 middle frontal process. The cheeks are derived from the 

 maxillary lobe, a projection sent 

 forwards from the region at 

 the base of the first branchial arch. 

 The mouth makes its first appear- 

 ance as a depression of the outer 

 layer of the embryo, which, at a 

 very early period, touches the cul- 

 de-sac of the alimentary tube; and a 

 perforation then takes place. The 

 roof of the mouth, after this, is 

 formed at first by the base of the 

 skull, as it continues to be in fishes 

 throughout life. The palate is a 



later formation, and is developed 



"magnlfiecTsix diameters! from tne maxillary lobes, which each 

 a, Nostril, and, beneath send a lamina inwards to meet its 

 it, the middle frontal fellow of the opposite side in the 

 process; b eye and be- middle line . Sometimes the union 

 neatli it, tne lateral fron- r ., , , , , , 



tal process; c, first bran- f alls to te completed, and the result 

 chial process, and, be- is a permanently cleft palate. The 

 tween it and the eye, groove over the upper lip, and the 

 the maxillary lobe; d, t of the j aw suppor ti n g the upper 

 ear: e, ventricular part f . ,'t T ? r 



of the heart; /, right mcisor teeth, are derived from the 

 auricle ; g, fore limb ; h, middle frontal process; and when 

 Wolffian body; i, place this fails to unite with one or both 

 where the allantois and max in ary i o ]bes, the result is a single 

 umbilical vesicle have , , / , \. 

 been torn across 5 h or double hare lip. 

 hind limb, 21& The process of separation of 



Fig. 154. EMBRYO LAMB, 



