DEVELOPMENT OF THE LEOPARD FROG 



413 



neural-fold stage into the tadpole involves a further elongation of 

 the embryo (Fig. 215) and a change in curvature of its dorsal 

 surface, which first becomes straight and then assumes an outline 

 that is sHghtly concave, while parts of the head become recog- 

 nizable anteriorly and the beginning of the tail is observed at the 

 postero-dorsal region. Although it is now covered by the ecto- 

 derm, the neural tube is still recognizable as a dorsal median 

 swelhng that expands anteriorly where the brain is beginning to be 



Sp.C 



Fig. 217. — Median section of an early tadpole of the frog (c/. Figs. 213 / and 



215 E). 



a, anus; br, brain; c, notochord; ep, epiphysis; h, heart; hy, hypophysis; in, infundihu- 

 lum; I, liver; m, mouth or stomodseum; sp.c, spinal cord; u.d., urinary duct. (After 

 Marshall from Ziegler.) 



formed. In the head region, various structures gradually make 

 their appearance. 



It will be recalled that in a stage when the neural folds were well 

 developed (Fig. 215 A) there was found anteriorly and ventrally 

 a thickened area of the ectoderm known as the sensory plate, and 

 posterior to this on either side another thickening, the gill plate. 

 As these plates become more clearly recognizable, there appears 

 on either side, in the dorsal part of the sensory plate, a slight 

 swelling, the optic vesicle (Fig. 215 C), marking the rudiment of 

 the eye, which is an outgrowth from the neural tube. Likewise, 

 the stomodceum, which is the ectodermal invagination forming the 

 mouth, appears as a median depression in the antcro-ventro por- 

 tion of the sensory plate; and the suckers, which are really glands 

 secreting a sticky mucus, as a pair of pits which eventuall}^ unite 

 as a ^'-shaped structure Ijang ventral and lateral to the mouth. 



