AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



the gut (splanchnopleure). When the embryo is spread out flat 

 on the yolk, the somatopleure and splanchnopleure belonging to 

 opposite sides diverge widely, but, as the embryo is folded oft', 

 they gradually approach, unite, and form a continuous boundary 

 to the body and gut respectively. The latter meets in front and 

 behind with inpushings of ectoderm that form the mouth-cavity 

 and posterior part of the cloaca (stomodaeum and proctodseum). 



Fig. 81. TRANSVERSE SECTION THROUGH EMBRYO DUCK AT THIRD DAY 

 (after Balfour}. The external layer is ectoderm; hy, endoderm; meso- 

 derm, a thick layer between the two; am, lateral amniotic fold; -so, 

 somatopleure ; sp, splanchnopleure (the reference letters wd and sb are 

 placed in co3lom); xp.r, spinal cord, at the side of which is section 

 through mesodermic somite; cA, notochord (the reference letters <>. 

 hy, sp are placed in the alimentary canal, the walls of which, at this 

 stage, are widely divergent). 



The first three visceral arches are supported by skeletal struc- 

 tures that become in the adult 



I. Mandibular. Quadrates and Meckel's Cartilages, Palatines and 

 Pterygoids. 



II. Hyoid, ) fuse ventrally f basi-hyal, 



III. Branchial 1, 



to form, 



basi- branchial, 



anterior cornua ; 



columellae. 

 posterior cornua. 



The last two visceral arches, corresponding to branchials 2 and 3 of the 

 frog, are small and have no skeleton. 



