92 STAGING OF AMPHIBIAN EMBRYOS 



STAGES IN THE NORMAL DEVELOPMENT OF HYLA REGILLA* 



StaRe #1^ : Length li/h mm,; neural folda high and unfused in cranial part of neurula but 

 in contact with each other in and posterior to region of hindbrain; presump- 

 tive optic vesicle indicated by intensely pigmented depression in ventrolater- 

 al wall of future forebrain. 



Stage #16 : Length 2 mm,; neural tube completely closed; sense and gill plates visible; 

 optic vesicle forming as shallow, thick-walled diverticulum of forebrain. 



Stage #17 •• Length 2^ mm. ; tallbud s.eparated from body proper by ventral notch; stomodeal 

 depression slight; nonmotile; optic vesicle finger-like outpocketlng. 



Stage #18 : Length 5 nim. ; tail about I/5 length of body; caudal fin appearing; suckers in- 

 dicated by two heavily pigmented areas Joined medially by narrow pigmented 

 band below stomodeum; beginning of muscular response to touch (simple flexure); 

 optic vesicle fully formed and consisting of thick distal wall, relatively 

 thin sides, and narrowing stalk. 



Stage #19 : Length 'j'-l/j nm. ; tail rounded, more than 1/3 length of body, nasal placodes 

 indicated by pigmented shallow depressions; suckers no longer Joined by pig- 

 mented band; coll response to touch; beginning of heart beat; optic vesicle 

 Invaginating to form optic cup; lens placode invaglnatlng into optic cup. 



Stage #20 : Length 5-l/5 nmi. ; tail less rounded, almost ^ length of body; appearance of 

 external gills, with circulation; melanophores appearing on dorsal part of 

 body; brief and weak swimming movements; optic cup fully formed and consist- 

 ing of thick, inner sensory layer and thin, outer tapetal layer, faintly pig- 

 mented at back of eye; lens vesicle formed but undifferentiated; optic stalk 

 disappearing. 



Stage #21 : Length 6 mm.; tail pointed, |- length of body; external gills branching; nasal 

 pits deep; sustained and strong swimming movements; lens vesicle faintly 

 visible through cornea; optic nerve beginning to form. 



Stage #22 : Length 65- mm.; tail longer than body; pigmentation on dorsal aspect of eye 



visible externally; retina differentiated into relatively narrow inner layer 

 of nuclei {ganglion\ cells) and wide outer layer of nuclei separated by narrow' 

 band of white matteA (inner molecular layer); rods and cones beginning to dif- 

 ferentiate outer parws; lens fibers forming; optic nerve well developed. 



Stage #23 : Length 7 nun,; proportion of tail to body about h to 5; operculum beginning to 

 form; colon differentiated and bent dorsally; eye completely pigmented; retina 

 further differentiated by formation of outer molecular layer separating nuclei 

 of rods and cones ( outer nuclear layer) from nuclei of bipolar neurones ( inner 

 nuclear layer); outer parts of rods and cones better differentiated, especial- 

 ly ellipsoid. 



Stage #2U : Length 1^ mm.; operculum covering gills; gut S-shaped; spacious pleuroperl- 

 toneal cavity ventral to colon; dorsal fin bowed; cavity of lens vesicle ob- 

 literated; outer segment, ellipsoid, and perhaps nyoid of rods and cones well 

 formed. 



* These stages, from #15> to §2h , differ slightly from those of Shumway for Eana plpiens 

 and are here reprinted with the kind permission of Dr. B. M. Eakin (From 19^*7, Univ. 

 Calif. Pub. Zool. 51:2l+5)- 



