APPENDIX 



Ckronolo^ical Summary of Or^an Anla^en 

 Appearance or Rana pipiens 



Stage 17: 3 mm. total body length. 



Blastopore closes, neurenteric canal transient, neural tube closed 

 but no differentiation of central nervous system, body elongated with 

 back curve changing from convex to concave, myotomes evident but 

 movement by surface ciliation. Internally most of major organ sys- 

 tems begin to appear including heart mesenchyme, visceral arches, 

 and lateral plate mesoderm, pronephros, hypochordal rod, sense and 

 gill plates, optic vesicles, and both auditory and olfactory placodes. 



Stage 18: 4 mm. total body length. 



First evidence of muscular movement, primary brain differentia- 

 tions, infundibulum and hypophysis still separate, lateral line nerve 

 develops from vagus placode, auditory placode becomes separated 

 from head ectoderm, lens placode formed, ventral roots develop from 

 spinal cord, hypochordal rod separated from gut, notochord becomes 

 vacuolated and acquires an elastic and a fibrous coating from sclero- 

 tome, oesophageal plug formed, dorsal aorta paired anteriorly and 

 single posteriorly, vitelline veins prominent and functional. 



Stage 19: 5 mm. total body length. 



Epiphysis well developed as evagination of prosencephalon, in- 

 fundibulum, and hypophysis are approximated in position, thyroid 

 evagination develops, trigeminal nerve from first cranial crest and 

 placode, facial and auditory nerves from second cranial crest and 

 placode, lens vesicle separated from head ectoderm, 45 pairs of 

 somites developed (by 5.5 mm. stage), 32 of which are in tail, muscles 

 with fibrillae, sclerotome abundant, heart differentiated and beating. 



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