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Io Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
expanded laterally; the midbrain projects a little from the front of the head. The groove separating fore- 
and mid-brain is best seen in ventral view (Fig. 22a). The hind-brain is dilated, and its roof is semi- 
transparent. The olfactory pits appear as a pair of slight depressions lying between the optic cups and 
the lateral walls of the fore-brain swelling. The optic cup is faintly outlined; it is almost circular and 
nearly 0.5 mm. in diameter. The edges of the lentic plate are sharply outlined, and there is a distinct 
central lentic pit. The mandibular somite forms an ill-defined swelling over the optic cup. The trigeminal 
crest appears as a raised ridge along the posterior border of this swelling. The facial-acoustic crest 
appears as a faintly outlined plate, anterior to the otocyst. The otocyst is broadly oval in outline and its 
opening forms a narrow cleft extending backward and upward. The glossopharyngeal crest forms a distinct 
raised plate immediately behind the otocyst. The vagus crest is distinctly separated from the glosso- 
pharyngeal. From its dorso-posterior angle a narrow ridge, caused by the dorsal ganglionic commissure, 
extends backward above the first three somites. Five gill clefts are present — the first two open to the 
pharynx. The mouth is a narrow cleft 0.5 mm. in length and 0.1 mm. in width. Fifty-nine somites are 
distinguishable externally. The Wolffian duct extends below the twelfth to thirty-sixth segments inclusive. 
The median ridge extends backward from the first somite, around the tail, and for 1.2 mm. along the ventral 
median line. 
Figs. 23, 23a, 23b. Embryo 9.0 mm. in length. Normal plate series No. 23. H.E.C. 1495. Lateral, 
dorsal and ventral views. X 8. 
The yolk stalk has an approximate antero-posterior diameter of 2.0 mm. The back line of the 
embryo is almost straight to within 1.5 mm of the end of the tail. The tail ends in a slightly expanded 
knob. The primary head bend is a right-angled one. The forebrain is bent at an angle of 45° to the 
front of the midbrain. The optic vesicles project prominently from the sides of the head. They are 
broadly oval in outline. There is a distinct central lentic pit. The mandibular and hyoid somites are 
somewhat collapsed and the boundaries of their dorsal expansions appear as distinct raised ridges. The 
otocyst causes a prominent oval swelling over the hyoid arch. The swelling immediately behind it is 
probably caused by both the glossopharyngeal and vagus crests. The first three gill slits are open at their 
dorsal ends. The fourth and fifth slits are established but do not open to the pharynx. The mouth opening 
is elongately lenticular in outline; its length is 0.5 mm., its greatest width 0.I mm. The heart is barely 
visible through the walls of the pericardium. The ventral wall of the oesophagus, and anterior wall of 
the vitelline duct, appear as a distinct curved line above and behind the heart. The cloaca forms a very 
slight swelling on the ventral surface of the body, 1.3 mm. anterior to the tip of the tail. Sixty-three pairs 
of somites appear externally. The last of the somites is hardly distinguishable in the expanded tail-knob. 
A median fold extends from the first somite backward along the dorsal median line, around the tail, and 
forward along the ventral median line to the cloaca. 
As compared with the preceding embryo of 7.5 mm., the main changes in this embryo are: the form- 
ation of an angle of 45° between the fore-brain and mid-brain; the expansion of the fore-brain; the breaking 
through to the pharynx of the third gill cleft; the absence of the dorsal flexure; the expansion of the tail 
knob, and the increase by fourteen of the number of pairs of somites visible externally. 
Figs. 24, 24a, 24b. Embryo 11.5 mm. in length. Normal plate series No. 24. H.E.C. 206, 207, 208. 
Lateral, dorsal, and ventral views. X 5. 
These figures are of an embryo of the same length and stage of development as those used in the 
tables for the internal structure. 
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