158 EMBEYOLOGY OF THE LOWEE VEETEBEATES en- 

 large vessel where the quantitative relation of surface to volume in 

 the blood-vessel is at its minimum but rather where there is present 

 a rich superficial network of capillaries, in which the ratio in question 

 is at its maximum. 



(II.) Visceral Clefts. — The visceral clefts develop in what 

 appears to be the most archaic method in Lampreys and Elasmo- 

 branchs where each arises as a lateral pocket (visceral pouch) of 

 the pharyngeal wall which meets and fuses- with a, much shallower, 

 ingrowth of the ectoderm, the apposed portion of endoderm and 

 ectoderm breaking down so as to bring about a free communication 

 between pharynx and exterior. Each cleft thus consists of a, usually 

 much larger, inner portion lined with endoderm and an outer portion 

 lined with ectoderm. 



The most frequent type of modification of this probably primitive 

 mode of cleft development is that so usually met with in the develop- 

 ment of hollow organs, namely that the cleft-rudiment, instead of 

 being a hollow pouch from the beginning, is for a time in the form of 

 a solid lamina of endoderm, which only at a later period develops a 

 cavity in its interior and becomes an open cleft. This modification 

 is found in Telepstomatous fishes, Lung-fishes and Amphibians. 



In the young Elasmobranch the gill-clefts are at first long slits 

 traversing the whole dorsi-ventral extent of the lateral wall of the 

 pharynx. Each septum or arch grows back at its outer edge to form 

 a valvular flap overlapping the cleft next behind it. In most cases 

 this backgrowth fuses with the next septum at its dorsal and ventral 

 ends so as to reduce the external opening of the cleft to a compara- 

 tively small dorsi-ventral extent. 



In all Gnathostomes, excepting the typical Elasmobranchs but 

 including the Holocephali, the hyoidean backgrowth becomes greatly 

 enlarged to form the operculum which overlaps the whole series of 

 clefts behind it. Correlated with this the outer portions of the sub- 

 sequent septa with their backgrowths become reduced. In these 

 cases we frequently find a marked tendency for the edge of the 

 opercular backgrowth to become fused with the body so as to restrict 

 the size of the opening behind it. Thus in the Eel the opercular 

 opening becomes reduced to a small persistent ventral portion, while 

 in Symbranchus the same holds but in this case the two openings 

 have fused together to form a small ventrally placed median 

 pore. 



A similar condition to this occurs in the tadpole of Discogloisus 

 while in other Anura the persisting opening is displaced to the left 

 side. Finally in Amniotes the fusion of opercular margin with body- 

 wall takes place along its whole extent so that the branchial region 

 becomes completely enclosed (see Chap. X.). 



Spiracle. — The spiracle or hyomandibular cleft always shows a 

 considerable amount of modification. In Elasmobranchs its dorsal 

 portion alone becomes perforate, although fusion of the pouch with 

 the ectoderm takes place throughout its whole dorsi-ventral extent. 



