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CHRONOLOGICAL SUMMARY OF ORGAN ANLAGEN 



Posterior choroid plexus 

 Rhombocoel 



Auditory capsule (cartilage) 

 Cranial nerve VIII 



Semicircular canals (inner ear) 

 Stapedial plate 

 Columella 



Annulus tympanicus 



Tympanum 



Middle ear vestibule 

 Perilymph space 



Eustachian tube 

 (hyomandibular pouch' 



Head mesenchyme 



Cranial cartilage 



Heart mesoderm 

 Parts of the middle and inner ear of the frog, schematized drawing. 



Stage 20: 6 mm. total body length. 



Hatches, oral suckers at maximum development, blood system well 

 developed, including endocardium, myocardium, and pericardium 

 of the heart, pulmonary veins develop but do not function, gill circu- 

 lation abundant, blood islands numerous, lymph hearts first appear 

 at level of somites III and IV, pronephros with glomi and segmental 

 duct become partially embedded in posterior cardinal veins, a solid 

 rod of ectoderm cells extend from olfactory pit to pharynx, four cra- 

 nial ganglionic masses appear, optic nerve develops from neuroblasts 

 of retina, and sympathetic nervous system arises from neural crests. 



Stage 21: 7 mm. total body length. 



Transition from larva to tadpole, mouth open, cornea transparent, 

 cerebral hemispheres and vesicles differentiated, endolymphatic duct 

 open to surface, heart folding upon itself, paired and metameric 

 dorsal root ganglia, interatrial septum separates auricles, sex cell 

 (genital) ridge appears. 



