THE GILL- CLEFTS. 159 



In tadpoles of 3 mm. length there are three pairs of gill- 

 pouches present, which appear almost simultaneously ; and by 

 the time of hatching of the tadpole two additional pairs are 

 formed behind these, making five pairs in all. 



The condition at this stage is well shown in the horizontal 

 section given in Fig. 74. The gill-pouches form vertical 

 partitions, radiating outwards from the pharynx to the surface 

 epiblast. Each pouch is formed of a double fold of hypoblast, 

 the two layers of which are in close contact with each other. 

 The outer ends of all five pairs of gill-pouches reach the epiblast, 

 and fuse with its inner or nervous layer. 



Of the five pouches of each side, the most anterior one is 

 the hyomandibular pouch or cleft (Fig. 74, HM), and the succeed- 

 ing ones are named first, second, third, and fourth branchial 

 pouches respectively : the hindmost or fourth branchial pouch 

 (Fig. 74, HC.4) is smaller than the others, and is often imper- 

 fectly developed at this stage. 



The parts of the wall of the pharynx between the successive 

 gill-pouches are spoken of as the visceral or gill arches. The 

 arch between the hyomandibular and the first branchial pouches 

 is named the hyoid arch (Fig. 74, HY) ; and then in succession 

 come the first branchial arch, BR.I : second branchial arch, BR.2, 

 and third branchial arch, BR..S. Behind the fourth branchial 

 pouch, HC.4, is an imperfectly defined fourth branchial arch. 



The pharynx is widest opposite the first branchial arches ; 

 and between the pair of fourth branchial arches it passes back 

 into the narrow oesophagus. 



About the time of formation of the mouth, the two hypo- 

 blastic lamellas, of which each gill-pouch consists, separate from 

 each other, so as to form a narrow vertical slit, or chink, leading 

 from the pharynx to the exterior. These slits are the gill- 

 clefts. 



The first clefts to open in this way are the second and third 

 branchial clefts, i.e. the ones immediately behind the first and 

 second branchial arches respectively. At a slightly later stage 

 the first branchial cleft, between the hyoid and first branchial 

 arches, also opens in a similar way ; and later still the fourth, 

 or hindmost branchial cleft opens. 



The hyomandibular pouch, although it is in its early stages 

 exactly like the hinder branchial clefts, and is fused in similar 



