546 



THE EMBRYOLOGY OF THE FROG 



Order 3. Amphibia (vertebrates with aquatic larvae, but usually 



air breathing in the adult condition), (Fig. 315). 

 Order 4. Reptilia (cold-blooded, air-breathing vertebrates). 

 Order 5. Aves (birds, feathered vertebrates). 

 Order 6. Mammalia (beasts or quadrupeds). 





A. B. C. 



II. 



Fig. 314. 

 HEMICHORDATA. 



I. Various types of Enteropncusta which are relatives of Balanoglossus. A, 

 Balanoglossus clavigerus; B, Glandiceps hacksi; C, Schizocardium brasiliense ; D, 

 Dolichoglossus kowalevskii; a, anus; ab, abdominal and caudal regions; b, 

 branchial region ; c, collar ; g, genital region ; gp, gill-pore or branchial cleft ; 

 gn, genital wing ; h, hepatic region ; m, position of mouth ; p, proboscis ; t, 

 trunk. (From Newman, A, B, C, after Spengel, D, Bateson.) 



II. A and B Rhabdopleura, C Cephalodiscus dodecalophus. A and C, entire 

 animals. B, diagram of median longitudinal section of A. A, a, mouth ; b, anus ; 

 c, stalk of zooid ; d, proboscis f e, intestine ; /, anterior region of trunk ; g, one 

 of the tentacles. (After Ray Lank ester. ) B, a, arm; an, anal prominence; col, 

 collar ; col.ne, collar nerve, c.s, cardiac sac ; int, intestine ; m, mouth ; ntc, noto- 

 chord ; oe, oesophagus ; pr, proboscis ; pr.c, proboscis-coelom ; ret, rectum ; st, 

 stomach; te, tentacles; tr.c, trunk-coelom ; v.n, ventral nerve. (B, after Sche- 

 potieff; C, after Mclntosh.) 



The frog is usually considered a transitional form separating the 

 lower from the higher craniata both embryologically and anatomically. 

 And although the craniates vary considerably among themselves, the 

 frog has enough in common with all such variations to make it a norm 

 or standard type for constant and valid reference, both as to anatomy 



