CH. XIII] ORGANS FROM THE EXDODERM 145 



is the hyomandibular cleft, and this is followed successively 

 by the first, second, third, and fourth branchial clefts. The 

 last is the smallest and is often imperfectly developed at this 

 time. 



The visceral or gill-arches lie between the clefts. The first 

 arch between the hyomandibular and the first branchial .clefts 

 is the hyoid arch (Fig. 41). Then follow the first branchial 

 arch (BK 1 ), second branchial arch (BR 2 ), and third branchial 

 arch (BR 3 ). Behind the fourth branchial pouch there is an 

 imperfectly defined fourth branchial arch. 



When the tadpole leaves its jelly-capsule, the pouches are 

 still double-walled, solid partitions ; but about the time when 

 the mouth forms, the endodermal lamella? of some of the 

 pouches separate and place the cavity of the pharynx in com- 

 munication with the exterior. The second and third branchial 

 clefts open first. Later the first branchial cleft opens, and later 

 still the fourth. 



The hyomandibular cleft is at first like the others, but it never 

 opens to the exterior. After its formation it separates from 

 its ectoclermal connection, and recedes from the surface. The 

 lamella? separate, and the cleft appears as a diverticulum of the 

 pharynx. 



Two other structures arise from the walls of the pharynx 

 shortly before the hatching of the tadpole. " The lungs arise 

 as a pair of pouch-like diverticula of the walls of the oesophagus. 

 They are at first exceedingly small and have strongly pigmented 

 walls." 



The thj-roid body appears about the time of hatching as a 

 short median longitudinal groove along the wall of the pharynx. 

 " The groove is shallow anteriorly, but deepens at the hinder 

 end, where it leads into a small conical pit-like depression of 

 the endoderm, forming the pharyngeal floor, just in front of 

 the pericardial cavity. Soon after the mouth opens, the thyroid 

 separates completely from the floor of the pharynx, remaining 

 as a solid rounded mass of pigmented cells, in close contact 

 with the anterior wall of the pericardium." 1 



1 Marshall ('93). 



