EARLY ORGANOGENY 131 



growths) which will be recognized as the external evidences of the 

 internally enlarging optic vesicles or opticoels. 



Visceral Arches 



Parallel to the posterior margins of the sense plate there develops 

 another thickened pair of elevations, directed forward. These are 

 the gill plates which merge imperceptibly with the more posterior 

 lateral folds of the sense plate and the closed neural folds. They 

 contain the ninth and tenth cranial nerve ganglia and represent the 

 forerunners of the gill or branchial arches. These are very important 

 in the development of the external gills used for aquatic respiration in 

 the tadpole. These gill plates acquire vertical grooves (furrows) 

 which separate the intermediate bulges into three prominent vertical 

 thickenings known as the visceral or branchial arches. The most 

 anterior of the grooves appears between the mandibular arch of the 

 sense plate and the first thickening of the gill plate and is known as 

 the hyoniandibular groove or furrow. This groove never really opens 

 through from the outside to the pharynx as a cleft. Just posterior to 

 this hyomandibular groove is the first gill thickening, the beginning 

 of the second visceral or hyoid arch which will provide the meso- 

 dermal structures to the tongue and operculum. The fifth groove, the 

 most posterior of all, is the next to develop. Then follow the third, 

 fourth, and sixth visceral grooves (the fifth developing later), divid- 

 ing the gill plate into vertical thickenings. The sixth is rudimentary 

 and posterior to the gill plate. The visceral grooves appear in the 

 sequence of I, II, III, IV, VI, V, counting from the anterior. The 

 intermediate thickenings between the grooves are the visceral arches 

 which remain rather solid, some of which will soon give rise to the 

 external gills. Those which give rise to gills are sometimes referred to 

 as branchial arches, the first of which is the third visceral arch. 



The visceral arches, from the mandibular as the first, may be 

 numbered in sequence in a posterior direction, and may be designated 

 as visceral arches I to VI. However, since the third visceral arch be- 

 comes the first branchial arch because it is the most anterior arch to 

 give rise to an external gill, to name the arches "branchial" one must 

 begin with visceral arch III (called branchial arch I) and number 

 them posteriorly as branchial arches I to IV. 



The term "visceral arch" is used because of the homology of 

 this structure with similar structures in other vertebrates, even though 



