332 



COMPARATIVE ANATOMY 



internal gills developed in adaptation to special conditions. They have 

 no genetic relation to any human structure. 



Development of Gills. Gill sUts develop from a series of paired endo- 



dermic diverticula of the pharynx which 

 meet corresponding invaginations of the 

 ectoderm. By the disappearance of the 

 double membrane thus formed the 

 pouches are converted into gill sUts. 

 The branchial arches develop from the 

 regions between the gill sUts. Each arch 

 has an endodermal pharyngeal lining 

 and an external ectodermal covering. 

 The core of each arch is mesodermal. 

 The difhculty of distinguishing the 

 boundary between ectoderm and endo- 

 derm of each branchial arch has re- 

 sulted in a difference of opinion in 

 regard to the origin of the gill lamellae. 

 Some embryologists assert that these 

 are ectodermal, while others claim that 

 they are endodermal. Sewertzoff (1916) 

 distinguishes " endobranchiate " cyclo- 

 stomes from " ectobranchiate " gnatho- 

 stomes. The issue does not appear to be 

 important since Cook (192 1) has found 

 that the pharyngeal endoderm of fishes 

 produces such characteristic ectodermal 

 structures as taste-buds and placoid 

 scales. Consequently we are forced to 

 conclude that the contrast in the 

 potencies of the two germ layers is not as 

 great as has sometimes been assumed. 



The levator and depressor muscles of 

 the gills are developed from the hypo- 

 meric mesoderm enclosed in each 

 branchial arch. The connective tissue, 

 the cartilage or bone, and the blood 

 vessels of each arch are derived from the 

 mesenchyma. The origin of this 

 mesenchyma has been a subject of controversy, but the evidence favors 

 the opinion that the mesenchyma of the branchial arches is proliferated 

 from both mesoderm and ectoderm, not exclusively from the mesoderm as 

 was once supposed. 



Fig. 277. — Pharyngeal region of 

 a young Squalus embryo, b, 

 bloodvessels; c, coelomic cavities of 

 gill arches; g, developing gills; gc, 

 gill clefts; h, hypophysis; m, mouth; 

 n, notochord; o, oculomotor nerve; 

 oe, esophagus; p, peritoneal cavity; 

 s, spiracular cleft; I— III, first to 

 third head cavities (I and III reversed 

 in figure). (From Kingsley's " Com- 

 parative Anatomy of Vertebrates.") 



