I.] THE FROCx. 171 



responding pouch, and usually lodging a large 

 blood-vessel {aortic arch) seen in section. 



c. The external branchice. These will be seen in 

 some of the sections as filiform outgrowths (from 

 I to 3 in number on either side) of the visceral 

 arches. 



Each consists of a mesoblastic core invested in 

 epiblast. If the five visceral pouches are seen, 

 the first gill will be found to arise behind the 

 second one (first branchial). 



k. Examine the lower sections of the above series 

 and look for the formation of the mouth cavity. 



a. The mesefiteron ; cf. supra; it ends blindly in 

 front and is lined by a non-pigmented epithelium 

 (yellow if prepared as directed and not stained). 



p. The stomodcBum; a sac-like involution of the epi- 

 dermis abutting against a, spacious, and well 

 marked on account of its dense pigmentation. 

 Its enclosed cavity is the mouth cavity. 



/. Compare similar sections through the head of an 

 older larva, in which the internal branchiae are 

 present. 



a. The visceral pouches ; those of the branchial 

 series are now in open communication with the 

 exterior [visceral clefts). 



ji. The tftternal branchice ; vascular pectinations of 

 the opposite faces of a. Note their number and 

 arrangement. 



y. The operculum; a backwardly-directed fold 

 arising behind the first pouch (from the hyoid 

 arch • cf. i. 7n). 



