144 



DEVELOPMENT OF THE FROG'S EGG [Cii. XIII 



OS 



HM 



BRl 



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



older embryo as a result of cell-division. 



The first three 

 pairs of gill-slits 

 appear almost si- 

 multaneously ; the 

 first two, however, 



IN / s^. W f s$P\ """" Before the third. 



HY 4^^^ ^^Ul HB When the tadpole 



H i leaves its capsule, 

 H2 there are five pairs 

 3 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 stage. 

 "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." 1 The most anterior pouch or cleft 



AR 



FIG. 41. AR. Archenteron. BRl, BR 2 , BR3. Branchial 

 arches. Hi, H 2 , R3. Gill-clefts. HB. Hyoid cleft. 

 HM. Hyomandibular cleft. HY. Hyoid arch. IN. 

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

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



1 Marshall ('93). 



