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8 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
this ridge. The glossopharyngeal-vagus nerve crest forms a faintly outlined triangular plate just posterior 
to the otic pit. There are distinct elongately oval plates over the first and second gill pouches. The gill 
clefts appear as narrow grooves in these plates. The ventral wall of the oesophagus and the anterior wall 
of the yolk stalk form a continuous curved line extending from the posterior end of the pharynx to the 
blastoderm. The heart appears as a faintly outlined straight tube Iying below the anterior part of this line. 
Tbirty somites are visible. The unsegmented axial mesoderm forms a ridge extending from the last somite 
nearly to the tip of the tail. A similar ridge extends forward from the first somite to the anterior edge of 
the otic pit. The Wolffian duct appears as a small ridge, 0.5 mm. in length, lying just ventral to the 
seventh to eleventh segments, inclusive. 
The more important changes in this embryo of 4.8 mm., as compared with the preceding one of 
3.8 mm., are the following: the head and tail regions have become straightened until more nearly in line 
with the trunk; a marked dorsal flexure is present; the cephalic nerve crests are more prominent and a 
shallow otic pit is present; the gill clefts are more prominent; the number of somites visible externally is 
increased by two. 
Fig. 19. Embryo 5.83 mm. in length. Normal plate series No. 19. H.E.C. 1497. Lateral view. X 8. 
The embryo lies with the right side towards the blastoderm. The connecting stalk between the 
embryo and blastoderm has an antero-posterior diameter of2.5 mm. The dorsal flexure is pronounced. The 
head is bent ventrally nearly at right angles to the trunk. The tail is bent but little ventrally. The optic 
vesicle is prominent and broadly oval in outline. There is a shallow groove separating the fore-brain from 
the hind-brain. The roof of the hind-brain is thin and translucent. The outline of the trigeminal nerve 
crest proper cannot be distinguished, but the boundaries of the thalamic and mandibular branches are 
faintly visible. The facial and glossopharyngeal nerve crests appear as prominent ridges extending from 
the dorsal zone of the hind-brain into their respective arches. The vagus crest forms an ill-defined triangular 
plate, separated anteriorly from the glossopharyngeal crest by a distinct groove. The otic pit is broadly 
oval in outline: it is deepest dorsally and anteriorly. There are three distinct gill clefts, none of which 
are broken through. The first gill cleft is almost twice the length of the third. The outline of the heart 
can be seen indistinctly through the translucent pericardium. Thirty-five or -six somites are visible externally. 
The Wolffian duct is seen as a faint ridge below the eleventh to sixteenth somites inclusive. There is a 
distinct median ridge extending around the tail. 
As compared with the preceding embryo 4.8 mm. in length, this specimen shows the following 
differences: the fore-brain and optic vesicles are more expanded; the pharyngeal region is longer and 
narrower; the third gill cleft is present; the otic pit is deeper; the glossopharyngeal crest forms a ridge 
distinct from that of the vagus crest; the dorsal flexure is more pronounced; the number of somites is 
increased by five or six. 
Figs. 20, 20a, 20b. Embryo 5.2 mm. in length. Normal plate series No. 20. H.E.C. 1355. Lateral, 
dorsal and ventral views. X 8. 
The embryo has been detached from the blastoderm, and is seen in Fig. 20 from the left side. The 
yolk stalk has an antero-posterior diameter of 1.7 mm. There is a decided dorsal flexure over the yolk 
stalk. The head is bent nearly at right angles to the body, and the tail has the stiff curve which seems 
characteristic of embryos about this age. The optic vesicles are oval in outline when seen from the side, 
and almost semicircular when seen from above or below. Above them is seen a shallow groove separating 
