THE DEVELOPMENT OF VERTEBRATA. 241 



respiration begin together. The external gills begin to 

 shrivel and disappear as the internal gills vascular out- 

 growths from the slits, covered by hypoblast are developed. 



The vessels in the mandibular and hyoidean arches which 

 were never more than rudiments soon disappear, but both 

 afferent and efferent arteries are now fully developed in the 

 other four arches. It should be noticed that the arrange- 

 ment of these arteries is decidedly simpler than that in the 

 dogfish ; there is simply one afferent and one efferent vessel 

 in each arch (contrast fig. 92), and these are connected by a 

 series of loops in the gill-filaments. 



Presently a new structure appears, not represented in any 

 of our other types, but existing in most ordinary fishes. 

 This is the opercular fold, an external fold developed from 

 the hyoid arch and extending back to cover in the gills 

 (fig. 123, right side). Presently the edge of this fold fuses 

 with the body-wall and only a small aperture is left, on 

 the left side, so that the gill -slits open, not directly to 

 the exterior, but into a chamber quite analogous (but 

 probably not homologous) to the atrium of Amphioxus. 

 The pore by which the water- current passes out evidently 

 is analogous with the atrial pore of Amphioxus.* 



15. Coelom and Pericardium. About the time of 

 hatching, two regions may be distinguished in the coelom, 

 corresponding to the two regions of the mesenteron. In the 

 intestinal region, owing to the great thickness of the yolk- 

 bearing mesenteron walls, the coelom is a mere narrow 

 chink (fig. 120), but it extends round all but the dorsal part 

 of the mesenteron. In the pharyngeal region there is no 

 yolk and the ccelom is larger, but confined to the ventral 

 region because elsewhere the formation of the gill-slits is 

 bringing epiblast and hypoblast close together. 



As the heart develops it projects more and more from the 

 floor of the pharynx into this expanded part of the coelom, 

 which is, therefore, the pericardium. As the Cuvierian 

 veins increase in size they form a transverse barrier 



" In one of the toads (Dactylethra) there is a pair of these open- 

 ings, right and left : this is obviously a more primitive condition ; 

 and the tadpoles of this genus are in other ways very primitive. 

 ZOOL. lb' 



