62 DEVELOPMENT OF THE FROG'S EGG [Ch. V 



beginning of the "suckers," or adhesive ghmds (Fig. 22, I)). 

 The nasal pits appear above the mouth (Fig. 22, fV 



The outlines of the three brain-vesicles can also be faintly 

 seen in surface view. A pair of swellings on each side of the 

 fore-brain shows the position of the eye-evaginations (Fig. 22, 

 D). In the pharyngeal region there first appears on each side 

 a vertical ridge, and later another ridge parallel to and behind 

 the first (Fig. 22, D, E). On these ridges gills appear as pro- 

 trusions of the surface, and later a third ridge. and gill are 

 formed behind and somewhat beneath the others. Some time 

 after hatching, the gill-slits break through to the exterior be- 

 tween the ridges or gill-arches, and at about the same time, 

 the mouth breaks through into the cavity of the pharynx. 



In the head-region the beginnings of some of the spinal 

 ganglia may be seen, and a series of mesodermal blocks also 

 appear and may be dimly seen from the outer surface. These 

 structures do not, however, appear as distinctly in surface views 

 of the embryo of Rana temporaria as they do in the embryos 

 of some other species. 



Soon after the nerve-tube has closed, the dorso-posterior end 

 of the body begins to extend backwards to form the tail (Fig. 

 23, D, E). The anal opening lies just behind and ventral to 

 this region of posterior growth. The anus seems to shift to a 

 more ventral position during the elongation of the tail. At 

 first the tail is a thick outgrowth of the posterior end of the 

 body, but as it grows longer it flattens from side to side, and 

 in later stages a thin fan-like border or fin develops on its 

 upper and lower margin (Fig. 38). 



