60G 



ANATOMY OF VERTEBEATES. 



numbers and increase of size : and as the fitness of the vascular 

 surface for respiration decreases, the developement of the gills 

 progresses. The branchial arches appear, three in quick succes- 

 sion, from behind forward, and branchial tubercles bud from them 

 in like order. The mouth and the branchial slits being now 

 opened, the arches move rythmically, so as to produce branchial 

 currents : the blood is yellowish, and the discs begin to show the 

 flat oval shape. As the vitellicle decreases, its circulation is 

 changed, or merged into the portal hepatic system ; and now that 

 through the branchial buds begins. The change of the vitelline 

 for the branchial circulation relates, in a general way, to that 

 from the confined to the free state of the young fish: but no 

 such alterations of the circulating or breathing systems attend 

 the escape of the fisli from the egg as mark extrication in the 

 Reptile and Bird, or birth in the Mammal. Vitelline respiration, 

 carried on in ovo by means of the imbibed water between the outer 

 and vitelline tunics, continues to operate for a longer or shorter 

 jieriod after the little fish is free, according to the species, and also 

 according to the indi'^'idual constitution in the same species. 



Each branchial bud is at first a solid cell-mass, and is excavated 

 to receive the blood with blood-discs in single file : secondary 

 tubercles bud forth at right angles to the primary ones, through 

 which similar blood currents flow : the primary buds become the 

 stems of the leaflets into which the secondary ones are developed, 

 and the cartilaginous axis of the arch and stems next appears. 

 The pseudo-branchia also shows itself behind the eye, in the form 

 of flattened elongated folds, through which the blood courses at 

 first in a few vascular loops. In the Anabas, and probably other 

 Labyrinthibranchs, the 

 epibranchial reservoir, 

 fig. 325, retains a com- 

 parative degree of sim- 

 plicity until the fish is 

 full-grown. 



The intestinal canal, 

 after the formation of the 

 mouth and vent, retains 

 its uniform diameter, ex- 

 cept where it is jiartly 

 surrounded by a mass of 

 the cells, in which the 



liver, fig. 424, /, is developed : the gall-bladder appears to be a ca^cal 

 piroduction from the intestine, independently of the liver. Opposite 



424 



Vore Part of enibvio Os; 



