Pr} 
Normal Plates of the Development of Squalus acanthias. 5 
is not so far advanced in this specimen as in the smaller one. In the larger specimen the tail swellings 
are somewhat farther differentiated from the blastodermic margin. 
Fig. 10. Embryo 3.1 mm. in length. Normal plate series No. 10. H.E.C. ıorI. Dorsal view. x 8. 
The medullary folds are elevated throughout their extent. They are curved towards the median 
line only in the posterior half of the trunk portion of the medullary plate. They are least elevated towards 
the posterior boundary of the cephalic plate. The anterior part of the cephalic expansion is turned ven- 
trally and a little to the right. The optic vesicles form a single deep depression in the cephalic expansion 
and the vesicle on the left side is seen to be a little invaginated laterally. The tail swellings project 
abruptly from the posterior rim of the blastoderm and extend backward 0.5 mm. over the yolk. 
As compared with the preceding embryo of 2.7 mm., this embryo shows the following changes: the 
optic vesicles are farther expanded; the edges of the cephalic plate are elevated; the tail swellings are 
farther differentiated from the blastoderm. 
Fig. ıı. Embryo 3.2 mm. in length. Normal plate series No. ıı. H.E.C. 978. Dorso - lateral 
view. X 8. 
The anterior third of the embryo is separated from the blastoderm. The free part is turned to the 
right and a little downward. The medullary folds in the head region are curved mesially but there is a 
large cleft between their free edges. They gradually approach and meet at the tip of the medullary tube. 
At the posterior limit of the former cephalic plate they approach each other and are in contact, although 
not fused, for a distance of 0.7 mm. behind this point. Posterior to this closed region the medullary canal 
widens until at the incisura neurenterica the folds are 0.3 mm. apart. The tail swellings project sharply 
from the rim of the blastoderm. They are 1.5 mm. in width at the base and extend backward 0.5 mm. 
over the yolk. 
As compared with the preceding embryo of 3.1 mm., the chief advance in development is in the 
more complete formation of the medullary canal. 
Fig. 12. Embryo 3.2 mm. in length. Normal plate series No. 12. H.E.C. 994. Dorsal view. X 8. 
The anterior sixth of the embryo is separated from the blastoderm and is turned a little to the left. 
The medullary folds are elevated and their edges are turned mesially throughout their extent. They are 
almost in contact just behind the anterior neuropore, and again behind the hindbrain. The optic vesicles 
are somewhat expanded laterally, although this is not well indicated in the figure. The first gill pouch is 
seen as a slight swelling on the left side of the embryo, just behind the free cephalic end. Ten pairs of 
somites are visible externally. On the left side the unsegmented axial mesoderm is seen as a distinct ridge 
extending backward 0.3 mm. beyond the last somite. The lateral boundaries of the archenteron form 
prominent ridges extending backward and outward on either side, from the sixth segment to the outer 
edges of the tail swellings. The tail swellings project backward over the yolk 0.5 mm. beyond the posterior 
rim of the blastoderm. 
This embryo is but little farther advanced than is the preceding one, which is of the same length. 
The chief evidence of farther development is the appearance of the first gill pouch in the present specimen. 
The closure of the medullary tube, as a whole, has proceeded somewhat farther, and the incisura neu- 
renterica is covered a little more by the medullary folds. 
