28 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
No. Ear Integument, Lateral Line Mouth Archenteron 
20 |Otic pit invaginated anteriorly and a little dor- | Like preceding stage (No. 19). Oral plate broken through anteriorly for —_ 
sally, forming a deep pocket which is flattened 60 u (by sections). Very thin posteriorly. 
laterally. Open part of otic pit broadly oval in 
outline, gradualiy flattening out posteriorly. In- 
vaginated part of otic pit lies over anterior end 
of first gill pouch. 
2I Otocyst 300 u in length. Vertical diameter equal _ Oral cleft 200 1 in length (by sections). — 
to two-thirds of length. Posterior third forms 40 » in width in broadest part. 
a broad neck connected with the skin ectoderm. 
22 Otocyst two-thirds as high as long. In thickest| Much as preceding stages (Nos. I9 and 20), ex- | Mouth an elongated oval slit, — 
part two-thirds as broad as high. Connected |cept for placodes (see cranial nerves). approximately four times as long as broad. 
with ectoderm by a neck occupying upper half 
of approximately the posterior third of outer 
wall. This neck directed slightly posteriorly and 
succeeded by a shallow groove in the thickened 
skin ectoderm. 
23 Otocyst nearly three fourths as high as long. Pharyngeal plate of epithelium sharply marked | Mouth opening oval in outline, three times _ 
off from general skin ectoderm dorsally, less so | as long as broad. 
ventrally ; 65 u thick in thickest portion. 2—4rows 
of nuclei. Extends posteriorly for 0.23 mm. (by 
sections) beyond last gill slit. Ectoderm over 
lower part of head 20 u. in thickness. 
24 Otocyst extends through fifty-seven sections of| A single layer of cuboidal or columnar cells | Mouth opening broadly oval in outline, — 
6 u, bluntly rounded posteriorly, roundly pointed |over head ventral to supra-orbital placode. In|reduced to a narrow cleft posteriorly. An- 
anteriorly, flatly rounded below. Ductus endo-|trunk region, a single layer of squamous or low |terior (dorsal) halves of mandibular arches 
Iymphaticus arises from a broad funnel-shaped | cuboidal cells, except over fin folds. Pharyngeal | bent mesially and enlarged. 
base, extends first dorsally, then dorso-poste- plate sharply marked off from general epithelium - 
riorly, fusing with skin above posterior end of dorsally. Supra-orbital, infra-orbital, mandibular, 
otocyst. and lateral-line placodes established. 
25 No much the same as in preceding stage |Much as preceding stage (No. 24). Much as preceding stage (No. 24). — 
(No. 24). 
26 |Otocyst 120 u in length (by sections); greatest Epithelium consists mainly of one layer of cells; 
| vertical diameter one-half of length, transverse 
|arises from the middle third of the dorsal surface 
diameter one-half of vertical. Epithelium of 
mesial and ventral walls much thicker than that 
of dorsal and lateral. Ductus endolymphaticus 
of otocyst, on the median side. 
almost directly posteriorly, fusin 
It extends 
with skin 
slightly in front of posterior wall of otocyst. 
two layers of cells on ventral surface of head, 
on sides and ventral surface of trunk. Pharyngeal 
plate much less definitely outlined than in pre- 
ceding stages. 
Mouth opening diamond-shaped in outline, 
almost as broad as long. Mandibular and 
maxillary processes bent at an angle of 
approximately 90°. Anterior ends of maxil- 
lary processes much enlarged — separated at 
median line by a cleft approximately one- 
third the width of the mouth opening. 
Very slight frontal process. 
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