6 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
Fig. 13. Embryo 3.3 mm. in length. Normal plate series No. ı3. H.E.C. 980. Dorsal view. X 8. 
The head and a little of the anterior part of the trunk of the embryo are free from the blastoderm 
and are turned to the right. The posterior two thirds of the embryo are unflexed, and are seen in straight 
dorsal view. The medullary folds meet for a distance of nearly 0.5 mm. in the region of the hindbrain; 
throughout the remainder of their length they are turned mesially and are almost in contact. The incisura 
neurenterica is covered in part by the medullary folds. Eleven pairs of somites can be seen externally. 
The lateral boundaries of the archenteron appear as distinct lines passing from the last pair of somites to 
the angle formed by the tail swellings and the posterior rim of the blastoderm. The tail swellings project 
backward over the yolk for 0.5 mm. Their lateral edges meet the posterior rim of the blastoderm nearly 
at right angles. 
As compared with the preceding embryo of 3.2 mm., this embryo of 3.3 mm. shows the following 
changes; the embryo is separated a little more from the blastoderm; the medullary folds are much higher 
and have fused over a small part of the medullary canal; the archenteron is reduced in width posteriorly, 
as is indicated by the position of the ridges formed by its lateral boundaries; the tail swellings are narrower, 
longer, and they join with the blastoderm at a more acute angle. 
Fig. 14. Embryo 3.5 mm. in length. Normal plate series No. 14. H.E.C. 982. Dorsal view. X 8. 
The head and a little of the pharyngeal region are separated from the blastoderm. The medullary 
folds are in contact, or nearly so, throughout their extent, except over the incisura neurenterica and at 
the anterior neuropore. Only the posterior end of the latter opening can be seen in the figure. The right 
optic vesicle appears as a very slight protuberance from that side of the head. Just behind this, and 
separated from it by a slight depression, is a faint elevation caused by the first gill-pouch. Only nine 
pairs of somites are shown in the illustration, although fifteen pairs can be counted in the sectioned 
embryo. The incisura neurenterica is covered in part laterally by the medullary folds, but still appears 
as a deep V-shaped opening when seen from above. 
As compared with the preceding embryo of 3.3 mm., the most noticeable advances in development 
shown by this specimen are the separation of the head from the blastoderm, and the partial enclosure of 
the incisura neurenterica. 
Fig. 15. Embryo of 3.25 mm. Normal plate series No. ı5. H.E.C. 1499. Lateral view. X 8. 
The embryo lies with the left side toward the blastoderm. The head and pharyngal regions are 
raised and free from the extra-embryonic disc. The medullary folds of the head region are in contact, 
except at the anterior neuropore and over the hind-brain. The optic vesicle forms a prominent hemispherical 
protuberance from the side of the head. Posterior to the optic vesicle is a slight swelling caused by the 
first gill pouch. The anterior mesoderm forms a faint ridge continuous behind with the somite ridge and 
extending forward to the optic vesicle. Fifteen pairs of somites are visible externally. The coelom is 
dilated and its walls are semi-transparent. The medullary canal is closed throughout the trunk region, and 
the incisura neurenterica is to a large measure enclosed. 
The more noticeable points of difference between this and the preceding embryo are: the bending 
of the head and separation of the anterior part of the embryo from the blastoderm; the increase in number 
of somites from ten to fifteen pairs; and the advance in the process of closure of the medullary tube and 
incisura neurenterica. 
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