132 NEURULATION AND EARLY ORGANOGENY 



external gills are formed. In the frog, visceral arches III to VI develop 

 external gills and they therefore can be properly called ''branchial 

 arches." A tabular comparison is given to clarify the distinction: 



Original Structure Structure Formed 



Visceral arch I Mandibular arch (jaw parts) 



" " II Hyoid arch (tongue and operculum) 



" " III Branchial arch I, first external gill 



" "IV " " II, second external gill 



V " " III, third external gill 



" "VI " " IV, rudimentary fourth external gill 



Three, sometimes four, of the vertical (visceral) furrows will open 

 through to the pharynx as clefts, functioning in gill respiration. They 

 open in the following order: visceral clefts III, IV, II, and V. The 

 presence and order of grooves therefore bears no relation to the 

 break-through order of the clefts. 



Following the hyomandibular groove in position, each is numbered 

 in sequence as a branchial cleft, when all are developed. Branchial 

 cleft I, for instance, is the slit from the outside to the pharynx just 

 anterior to branchial arch II and immediately following the hyoman- 

 dibular groove. Most of the arches are named for the gills to which 

 they give rise. This is true except for the first (mandibular) and 

 the second (hyoid). These do not give rise to gills but to jaw, tongue, 

 and opercular parts. The confusion of the terms visceral and branchial 

 need not be serious if we remember that all arches are visceral and 

 are numbered from the anterior, while the third visceral arch is only 

 the first branchial — i.e., the first to have gills. 



Posterior to the dorsal limits of the gill plate will be seen (at the 

 2.5 mm. stage) a slightly elongated swelling in the direction of the 

 embryonic axis. This is the surface indication of the internal enlarge- 

 ment of mesoderm known as the pronephros or head kidney. More 

 posteriorly and slightly dorsal to this level may be seen the < -shaped 

 surface indications of the internal, mesodermal myotomes, or muscle 

 segments. 



Origin of the Proctodeum and Tail 



At the posterior end of the embryo in the neurula stage the 

 neural folds converge, as do the lateral lips of the blastopore, so that 

 they become confluent. The originally oval blastopore becomes a 

 vertical slit, due to the active merging of the two lateral neural folds. 



