THE FOREGUT 197 



The finger-like and branched external gills arise as outgrowths of 

 the lateral wall of the third, fourth, and fifth visceral (branchial I, II, 

 III) arches shortly after the gill clefts are perforated. The first and 

 second external gills arise at the 5 mm. stage and the third at the 

 6 mm. stage of development. The arches carry with them the covering 

 ectoderm and the capillary loop of blood vessels and the nerves that 

 are always found within the arches themselves. The ectodermal cover- 

 ing is very thin so that the relatively large capillaries in each gill can 

 readily exchange CO._. and O.. in the surrounding aqueous medium. 

 These external gills constitute the respiratory organs of the larva from 

 about the fifth to the tenth days, and then the gills begin to atrophy. 

 This process of degeneration is assisted by the development of a 

 posterior growth from the hyoid arch, known as the operculum. This 

 is a membrane which covers the gills and forms, along with a similar 

 membrane of the other side, a ventral and ventro-lateral opercular 

 chamber. While the external covering of the operculum is ectodermal 

 it contains mesoderm. Water taken in through the mouth continues to 

 pass over the external gills, within the opercular chamber, but escapes 

 through the spiracle at the posterior margin of the left opercular fold. 



In the meantime the internal gills must develop in order to take 

 over the respiratory functions that gradually are being relinquished 

 by the external gills. These internal gills develop a double row of 

 filaments, ventral to the branchial arches and arising doubly from the 

 postero-external (anterior and posterior) faces of the same first three 

 pairs of branchial arches. These are visceral arches III, IV, and V. 

 The fourth pair of branchial (sixth visceral) arches may also give rise 

 to reduced single internal gills from their anterior faces only. These 

 gills are termed internal because of their position on the arches and 

 also because of the fact that they are covered by a body flap, the 

 operculum, and are therefore truly within the body. The entire oper- 

 cular (branchial) chamber is lined with ectoderm, however, and this 

 includes the covering of all of the external and internal gills. The 

 internal position of this new set of gill filaments is therefore secondary. 



The ventro-lateral position of the external gills, plus the develop- 

 ment of the operculum, tends to move them to a position beneath 

 rather than lateral to the pharynx, within the spacious opercular cavity 

 or chamber. The original endodermal branchial pouches, lateral to 

 the pharynx, become partially separated off by lateral projections of 

 the pharyngeal floor and also by longitudinal folds in the lateral aspects 



