144 



DEYELOPMEXT OF THE FROG'S EGG [Cii. XITI 



same in the older and younger embryos, or greater in the 

 older embryo as a result of cell-division. 



three 



gill-slits 



The first 

 pairs of 



appear almost si- 

 multaneously ; the 

 first two, however, 

 before the third. 

 When the tadpole 

 leaves its capsule, 

 there are five pairs 

 of gill-slits ; the 

 two new pairs have 

 appeared succes- 

 sively behind the 

 third. A horizon- 

 tal section through 

 the larva (Fig. 41) 

 shows to best ad- 

 vantage the five 

 clefts at this stag-e. 

 '^ The gill-pouches 

 form vertical par- 

 titions radiating- 

 outwards from the 

 pharynx to the 

 surface - ectoderm, 

 Each pouch is 

 formed of a double 

 fold of endoderm, 

 the two layers of 

 which are in close 

 contact with each 

 other. The outer 

 ends of all five 

 pairs of gill-pouches reach the ectoderm and fuse with its 

 inner or nervous layer, "i The most anterior pouch or cleft 



Fig. 41. — AR. Archenteron . BRi, BR2, BR3. Brancliial 

 arches. Hi, H\ m. Gill-clefts. HB. Hyoid cleft. 

 HM. Hyomaiulibular cleft. HY. Hyoid arch. IN. 

 Infundibulum. OF. Olfactory pit. OS. Optic stalk. 

 P. Pronephros. S. Segmeutal duct. (After Marshall.) 



1 Marshall ('93). 



