586 



THE EMBRYOLOGY OF THE FROG 



a.k.' 



The Hyoid Arch is the arch lying between the hyomandibular and I 

 branchial pouches. 



Branchial Arches, or Gill Arches, are the names given to the remain- 

 ing arches. 



The Branchial Clefts, or Gill Clefts, are the names given the various 

 pouches after they have opened to the surface of the ectoderm, and in- 

 ternally into the pharynx. The second and third clefts are the earliest 

 to become perforated, while the hyomandibular pouch does not become 

 perforated at all, but disappears shortly after the first perforations occur. 



The Eustachian tube 

 or Tubo-tympanic cavity 

 of the ear, forms from the 

 dorsal wall of the hyo- 

 mandibular pouch. 



The External Gills 

 (Figs, 337, 341), which 

 appear just before hatch- 

 ing, are small outgrowths 

 from the outer surfaces 

 of the dorsal ends of the 

 first and second branchial 

 (second and third vis- 

 ceral) arches. A small ex- 

 ternal gill also appears 

 later on the third bran- 

 chial arch. The two an- 

 terior gills grow rapidly 

 and form large lobed 

 processes by the time the 

 mouth opens. They be- 

 come vascular and form the first breathing or respiratory organs of the 

 tadpole. The posterior pair remain in a much more undeveloped state. 

 The Operculum (Fig. 341) is the name given to the covering of the 

 external gills. It makes its appearance before the mouth opens, growing 

 out from the posterior borders of the hyomandibular arches. These out- 

 growths extend backward so rapidly that by the time the anterior gills 

 have reached their maximum size, the operculum covers them in what 

 is called the opercular cavity. 



The right opercular fold becomes fused with the body, while the left 

 remains partly open to form the opercular tube or spiracle. 



The Internal Gills form as tiny elevations on the postero-external 

 faces of the branchial arches, just as the gill clefts are perforated. A 

 thin layer of ectodermal cells covers them, just as it does the external 

 gills. These gill coverings become doubled on the first three branchial 

 arches and remain a single row on the fourth branchial arch. 



Fig. 341. 



Ideal diagrammatic transverse section through a 13 mm. 

 frog tadpole, showing the first gill arch. 



The right side shows the external gills (a.fc.) extending 

 through the gill opening which is almost closed. As the 

 opening closes the gills draw inward. On the left side the 

 gill opening is wide open with the external gills showing 

 themselves free, a.b., the first gill artery ; a.k., external 

 gills ; ao, aortic roots ; aud, otic capsule ; c.e., external caro- 

 tid ; d, pharynx ; s, lumen of the intestine ; filtr, anlage of 

 the filter apparatus (gill rakers); h, heart ; i.k., internal 

 gills ; nr, anlage of the neural canal ; op, gill operculum ; 

 v.b.c., common branchial vein ; v.b.e., external branchial 

 vein, v.b.i., internal branchial vein. (After Maurer.) 



