DEVELOPMENT OF GILL RESPIRATORY ORGANS 641 



b. Development of Gills in Necturus maculosus 



The gills of Necturus arise at about the 10- to 14-mm. stage as fleshy 

 columnar outgrowths from a limited region of the third, fourth, and fifth vis- 

 ceral arches (i.e., the first, second, and third branchial bars or gill arches). 

 (See fig. 302C.) These outgrowths are at first conical in shape (fig. 227) 

 but later become compressed laterally. Epidermal outgrowths or gill filaments 

 arise from the sides of these outgrowing gill columns (fig. 302C, D). (See 

 Eycleshymer, '06.) As the larva grows and matures, the development of gill 

 filaments from the sides of the gill columns becomes profuse (fig. 302E). 

 During the elaboration of the gill column and gill filaments, the original aortal 

 (vascular) arch becomes separated into two main components, the afferent 

 artery from the ventral aorta to the gill column and an efferent artery from 

 the gill column to the dorsal aorta (Chap. 17). 



c. Development of Gills in the Larva of the Frog, Rana pipiens 



1) Development of External Gills. As stated on p. 639, two types of gills 

 are developed in the frog larva, external and internal. The external gills are 

 developed as follows: At about the 5-mm. stage, the gill-plate area on either 

 side of the embryo begins to be divided into ridges by vertical furrows (fig. 

 303A). Eventually, three ridges appear. These ridges represent the third, 

 fourth, and fifth visceral arches (i.e., the first, second, and third branchial 

 arches) . From the upper external edges of these arches, a conical protuberance 

 begins to grow outward, beginning first on the first branchial arch. Ultimately, 

 three pairs of these fleshy columns are formed (fig. 303B). From these gill 

 columns, finger-hke outgrowths, the gill filaments, arise. An abortive type of 

 gill may form also in relation to the fourth branchial arch. The gill column and 

 the filaments possess the ability to expand and contract. 



2) Formation of the Operculum. At approximately the 9- to 10-mm. stage, 

 an oro-pharyngeal opening is formed by rupture of the pharyngeal membrane. 

 At this time, also, the opercular membranes arise. Each operculum arises as 

 a fold of tissue along the caudal edge of the hyoid or second visceral arch. 

 This opercular fold on either side grows backward over the gill area. Even- 

 tually, the two opercula fuse ventrally and laterally with the body waU to 

 form a gill chamber for the gills (fig. 303C). On the right side the fusion of 

 the operculum with the body wall is complete. However, on the left side the 

 fusion of the operculum in the mid-lateral area of the body wall is incom- 

 plete and a small opening remains as the opercular opening (fig. 257B'). 



3) Internal Gills. During the above period of opercular development, the 

 external gills become transformed into internal gills, and branchial clefts form 

 between the gill arches. In doing so, the external gill columns gradually shrink, 

 and small, delicate, gill filaments sprout from the outer edges of the gill arches 

 (fig. 303D). External respiration is achieved now not by a movement of the 

 gill in the external medium, as previously, but by the passage of water into 



