68 STAGING OF AMPHIBIAN EMBRYOS 



c. Hyla Regilla 



STAGES IN THE NORMAL DEVELOPMENT OF HYLA REGILU\* 



Stage #15: Length 13/4 mm. ; neural folds high and unfused in cranial part of neurula but in contact with each other in and 

 posterior to region of hindbrain; presumptive optic vesicle indicated by intensely pigmented depression in ventro- 

 lateral 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 2-1/2 mm. ; tailbud separated from body proper by ventral notch; stomodeal depression slight; nonmotile; 

 optic vesicle finger-like outpocketing. 



Stage #18: Length 3 mm. ; tail about 1/3 length of body; caudal fin appearing; suckers indicated 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 tliin sides, and narrowing 

 stalk. 



Stage #19: Length 4-1/3 mm. ; tail rounded, more than 1/3 length of body, nasal placodes indicated by pigmented shallow 

 depressions; suckers no longer joined by pigmented band; coil response to touch; beginning of heart beat; optic 

 vesicle invaginating to form optic cup; lens placode invaginating into optic cup. 



Stage #20: Length 5-1/3 mm. ; tail less rounded, almost 1/2 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 

 consisting of thick, inner sensory layer and thin, outer tapetol layer, faintly pigmented at back of eye; lens vesicle 

 formed but undifferentiated; optic stalk disappearing. 



Stage #21: Length 6 mm.; tail pointed, 1/2 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 6-1/2 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 matter (inner molecular layer); rods and cones beginning to differentiate outer parts; lens fibers forming; 

 optic nerve well developed. 



Stage #23: Length 7 mm. ; proportion of tail to body about 4 to 3; 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, especially ellipsoid. 



Stage #24: Length 7-1/4 mm. ; operculum covering gills; gut S-shaped; spacious pleuroperitoneal cavity ventral to colon; dorsal 

 fin bowed; cavity of lens vesicle obliterated; outer segment, ellipsoid, and perhaps myoid of rods and cones well 

 formed. 



* These stages, from #15 to #24, differ slightly from those of Shumway for Rana pipiens and are here reprinted with the kind 

 permission of Dr. R. M. Ea kin (From 1947, Univ. Calif. Pub. Zool. 51:245). 



"The hen does nol produce the egg, bid ihc egg produces the 

 hen and also other hens . . . We know that the child comes from 

 the gcrfn cells and not from the highly differentiated bodies of the 

 parents, and furthermore , that these cells are not made by the 

 parents' bodies but these cells Itave arisen by the division of ante- 

 cedent cells . . . Parents do not transmit their characters to their 

 offspring, but these germ cells, in the course of long development, 

 give rise to adult citaracters similar to those of the parent. " 



E, G. Conldin in "Heredity and Environment" 



