THE GILL CLEFTS AND ARCHES. 135 



I. The Gill Clefts and Arches. 



Some little time before the tadpole is hatched a series of 

 vertical ridge-like thickenings appear on the sides of the head 

 and neck. These are the visceral arches, and are six in number 

 on each side. 



The most anterior is the mandibular arch, and gives rise 

 later on to the lower jaw: the second is the hyoid arch: 

 and the succeeding four are the first, second, third and fourth 

 branchial arches respectively. 



About the time of hatching the external gills grow out as 

 branching and richly ciliated processes from the outer surfaces 

 of the first and second branchial arches, and a little later from 

 the third branchial arches as well (Fig. 31). 



At the same time, the hypoblastic epithelium at each side of 

 the buccal cavity becomes thrown into folds, which extend out- 

 wards towards the surface of the neck as paired outgrowths, lying 

 between the visceral arches. Of these outgrowths or pouches, 

 which are known as visceral clefts, there are five on each side. 

 The most anterior one is the hyomandibnlar cleft, and lies 

 between the mandibular and hyoid arches : its outer end lies 

 very close to the surface of the neck, though it does not actually 

 open to the exterior. 



The four hinder visceral clefts perforate the skin about the 

 time of formation of the mouth opening, i.e., in tadpoles of about 

 9 mm. length, and open to the exterior as the gill clefts. These 

 are slit-like openings lying between the hyoid and first branchial, 

 first and second branchial, second and third branchial, and third 

 and fourth branchial arches respectively ; and are known as the 

 first, second, third and fourth branchial clefts. 



From the hyoid arches a pair of opercular folds arise, which 

 grow back over the external gills, and the branchial arches and 

 clefts. The two opercular folds meet below the neck in the 

 mid-ventral line, and enclose the gills in a branchial chamber. 

 The hinder borders of the opercular folds fuse with the body 

 wall except at one place on the left side, where a spout-like 

 opening remains through which the branchial chamber com- 

 municates with the exterior. 



As the opercular folds develope, the external gills gradually 

 shrivel up, and are replaced functionally by the internal gills. 

 These latter are delicate thin-walled vascular tufts, arranged in 

 a double row along the ventral half of each of the first three 



