AHC 



Text- fig. 1. — Reconstruction of the head of an embrj^o of Sqixalus acanthias 



4-5 mm. long seen from the left side. 

 Text-fig. 2. — Embryo 5 mm. long. First appearance of muscle-fibres in somite 5. 



The relation of somites to gill-slits and dorsal nerves is plainly seen. 

 Text-fig. 3. — Embryo 6 mm. long. The fifth somite is indistinct, the sixth is 



covered by the vagus except for the posterior dorsal corner, which begins to 



assume the hook-shaped appearance. 

 Text-fig. 4. — A single section through an embryo 5 mm. long, showing the single 



nature of the somite under the auditory capsule. 



Explanation of Lettering. 



Ab, Abducens nerve. A.H.C., Anterior head cavity of Piatt. Ap.. Auditory 

 Placode. A.S., Auditory Sac. F, Facial nerve. G, Gut. Gl, Glossopharyngeal 

 nerve. G.8. 1 -5, Gill-slits 1 to 5. H, Heart. N. 1-7, Neuromeres 1 to 7. 

 Oc, Oculomotor nerve. S. 1-10, Somites 1-10. Sp, Spiracle. Sp. Gn. 2, 

 Second Spinal Ganglion. Tr, Trigeminal nerve. V, Vagus nerve. V. 7-10, 

 Ventral nerve-roots of segments 7 to 10. V.C.L., Vena Capitis Lateralis. 



