4 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
of the archenteron are indicated on either side by a ridge on the blastoderm running outward and backward 
from the posterior margin of the cephalic plate to the point of union of the tail swellings and the margin 
of the blastoderm. 
When compared with the preceding embryo 1.8 mm. in length, the following differences are noti- 
ceable: the medullary plate is raised somewhat more above the surface of the blastoderm; the cephalic 
plate is expanded to a much greater extent and its boundary is sharply marked; the trunk portion of the 
medullary plate is longer and narrower than in the smaller embryo; the tail swellings are more clearly 
marked off from the posterior margin of the blastoderm and, by their extension backward on either side of 
the median line, form the walls of a distinct canal — the incisura neurenterica; the anterior tongue-like 
process of the archenteron is covered by the expanded medullary plate. 
Fig. 8. Embryo 2.5 mm. in length. Normal plate series No. 8. H.E.C. 1354. Dorsal view. x 8. 
The cephalic plate is broadly spatulate in outline and is sharply marked off from the trunk region 
of the medullary plate posteriorly. It is 0.9 mm. in length, broader before than behind, and has a maximum 
breadth of 0.6 mm. Its lateral edges are turned a little ventrally. The right edge is indented with four, 
the left edge with two “neuromeres” of Locy. The optic vesicles appear in the anterior part of the cephalic 
plate as a pair of depressions which are deep anteriorly and become shallower posteriorly. These depres- 
sions are separated by a distinct median ridge. The medullary folds are elevated throughout the trunk 
portion of the medullary plate. They curve outward rather abruptly on either side of the shallow incisura 
neurenterica. The tail swellings project backward 0.3 mm. beyond the posterior rim of the blastoderm. 
The lateral boundary of the posterior part of the archenteron can be traced on the right side only. It 
extends from the fifth intersegmental cleft to the point of union of the tail swelling and the margin of the 
blastoderm on that side. 
The more important changes shown by this embryo as compared with the preceding one 2.0 mm 
in length are: the great expansion of the cephalic plate and formation of definite optic vesicles; the 
elevation of the medullary folds along the entire extent of the medullary plate posterior to the cephalic 
expansion; the extension of these folds on either side of the incisura neurenterica; and the growth of the 
tail swellings. 
Fig. 9. Embryo 2.7 mm. in length. Normal plate series No. 9. H.E.C. 997. Dorsal view. X 8. 
The cephalic plate is broadly oval in outline. Its length is 1.0 mm., its greatest breadth 0.75 mm. 
The optic vesicles are confluent, forming a deep pit extending from the anterior edge nearly to the middle 
of the cephalic plate. The greatest width of the pit seen in a direct dorsal view is nearly 0.5 mm. Im- 
mediately behind the cephalic expansion the medullary plate is almost flat, but the medullary folds are 
elevated along the posterior half of the trunk part and extend outward on either side of the incisura 
neurenterica.. The median furrow is seen extending backward from the posterior edge of the optic pit. 
Posteriorly it is indistinguishable in the floor of the medullary canal. The tail swellings extend 0.4 mm. 
behind the margin of the blastoderm. They are 1.5 mm. in width at their base. The incisura neurenterica, 
when seen from above, is broadly V-shaped. It extends inward 0.3 mm. from the tips of the tail swellings. 
The lateral boundaries of the archenteron appear as faint ridges extending from the narrowest part of the 
medullary plate to the outer edge of the base of the tail swellings. 
The most noticeable advance in development of this embryo as compared with the preceding one 
of 2.5 mm. is the confluence and deepening of the optic vesicles. The formation of the medullary canal 
