14 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
This embryo is similar in general external appearance to the preceding one 20.6 mm. in length. In 
comparison with the latter, the most noticeable changes are: the change in shape of the nasal pits; the 
lengthening of the gill filaments; the extension of the pectoral fin; the extension posteriorly of the lateral 
line placode, and the broadening of the caudal fin. 
Figs. 30, 30a, 30b. Embryo 28.0 mm. in length. Normal plate series No. 30. H.E.C. 1357, 233, 221. 
Lateral view. X5. 
The fore-brain protuberance, when seen in direct ventral view, is almost square; it projects a little 
anteriorly. The dome-shaped mid-brain swelling projects forward nearly I mm. beyond the fore-brain. 
A shallow external groove marks the boundary between the mid- and hind-brain. The roof of the fourth 
ventricle is thin but not semitransparent. It is bounded dorsally by two lateral thickenings — the anlagen 
of the cerebellum. The mouth is approximately three times as broad as long — its upper boundary is 
obscured by the projection of the fore-brain swelling. The nasal pits are set at only a slight angle to the 
transverse plane of the head. On the anterior edge of each, the anterior flap of the nasal valve appears 
as a slightly thickened projection. The posterior flap forms a less well marked projection. The preoral 
canals appear as a pair of fine lines between the nasal pits, extending forward from the posterior end of 
the fore-brain swelling. At their anterior ends they separate and arch outward over the mesial edges of 
the nasal pits. Long gill filaments project from all the gill slits. The spiracular slit is less than a third 
the length of the remaining ones. The anterior and posterior dorsal fins are each about 0.6 mm. in height. 
The anterior fin is nearly 2.5 mm. in length at its base, and the posterior fin two-thirds of this length. The 
dorsal and ventral folds of the caudal fin are of equal height (0.6 mm.) and about 6 mm. in length. The 
pelvic fins are rounded projections 2 mm. in length and I mm. in breadth. The pelvic fins appear as a 
pair of rounded folds expanded posteriorly. Between their posterior ends is the elevated cloacal plate. 
The gut forms a prominent rounded ridge, extending forward some distance from the cloacal plate. 
The differences between this and the preceding embryo, 24.7 mm. in length, are not extensive. The 
chief ones are: the increased size of the fore-brain swelling and its projection forward, and the relatively 
shorter spiracular cleft. 
Fig. 31. Embryo 34.0 mm. in length. H.E.C. 363, 176, 353. Lateral view. X 5. 
The embryo has the same general shape as the preceding one, but is somewhat broader, both 
vertically and transversely, in proportion to its length. The mid-brain does not project so prominently, and 
there is no depression between the mid- and hind-brain swellings. The roof of the hind-brain is not 
distinctly outlined, as in the preceding specimen. The supra-orbital canal appears as a fine line above the 
eye. It becomes confluent ventrally with the ethmoid canal, which lies lateral to the nasal pit. The lateral 
line canal extends nearly to the end of the tail. A row of ampullae lie dorsal to it in the middle trunk 
region. The posterior and anterior dorsal fins are more rounded than in the preceding specimen and are 
much thickened at their bases. The posterior edge of each fin extends backward a little from the end of 
the base. The caudal fin is spatulate in outline. The highest part of the dorsal fold lies a little behind 
that of the ventral. 
Figs. 32, 32a, 32b. Embryo 37.0 mm. in length. N.T.S. No. 32. H.E.C. 363, 176, 353. Lateral, 
dorsal and ventral views. X 5. 
The growth of mesenchymal structures has obscured the outlines of the main divisions of the brain 
which have, up to this stage, determined the modeling of the head. From the ventral surface of the head, 
