160 OUTLINES OF CHORDATE DEVELOPMENT 



ol 



with which they fuse (Figs. 54, 57). These are the visceral 

 pouches. They appear very early and the formation of the first 

 two or three pairs was mentioned in the preceding chapter. 

 Altogether six pairs are formed, decreasing in size and impor- 



tance posteriorly. As these 

 pouches extend out to the ecto- 

 derm they divide the mesodermal 

 tissue lying between ectoderm and 

 endoderm, into a series of ver- 

 tical rods known as the visceral 

 arches. The most anterior visceral 

 pouch is the hyomandibular; in 

 front of it, between this and the 

 mouth, lies the mesodermal man- 

 dibular arch. The remaining 

 pouches are the first to fifth 

 branchial pouches. Between the 

 hyomandibular and first branchial 

 pouches is the hyoid arch, while 

 between successive branchial 

 pouches are the branchial arches. 

 Like the pouches these diminish 

 in size posteriorly until the last 

 is quite small and incompletely 

 marked. Some additional facts 

 FIG. 54. Diagram of a frontal regarding the last branchial pouch 



section of a frog larva at the time of r 



hatching. After Marshall (modi- will be mentioned later in connec- 

 * tion with the ultimobranchial 



lum; in, intestine; n, nephrostome; bodies. 



o, base of optic stalk; ol, olfactory . . 



pit (placode); p, pharynx; t, pro- About the time the mouth IS 



nephric tubules; II, hyoid arch; nr , pr , Ar l Qr Qnmj arm^ar within *>ar>Vi 



ni-vi, first to fourth branchial opened^ spaces appear witnm eacn 



arches; 1, hyomandibular pouch; branchial pOUch; these Cavities 

 2-6, first to fifth branchial pouches. -, .. ..-, .-, , 



become continuous with the phar- 



yngeal cavity, and soon perforate the area of ectodermal 

 and endodermal fusion, forming the branchial clefts or gill 

 clefts. The second and third clefts are perforated earliest, 

 and soon after, the first and fourth. The hyomandibular 



