| Normal Plates of the Development of Squalus acanthias. B 
of the blastoderm, with which it is continuous posteriorly. The anterior end of the plate is a little expanded 
_ I mm. in breadth — and is raised above the level of the blastoderm by the growth of the archenteron 
r [ie below it. A well defined median furrow (the primitive furrow of Locy) extends forward from the shallow 
rim notch. The anterior end of the furrow lies 0.4 mm. posterior to the anterior edge of the embryonic 
plate, and is somewhat expanded. 
Fig. 5. Embryo 1.6 mm. in length. Normal plate series No. 5. H.E.C. 988. Dorsal view. X 8. 
The medullary plate extends forward 1.o mm. from the thickened posterior margin of the blastoderm. 
It is a little broader anteriorly than posteriorly, being slightly constricted where it passes over into the rim 
of the blastoderm. The edges of the medullary plate are turned a little ventrally. Its greatest breadth is 
1.o mm. The rim notch forms a shallow depression at the median line of the margin of the blastoderm. 
From the rim notch a deep median groove extends forward for three fifths the length of the medullary 
plate. The fore-gut forms a broad tongue-like process which extends forward 0.2 mm. beyond the anterior 
edge of the medullary plate. 
In comparison with the preceding embryo of 1.4 mm. the most noticeable features of this embryo 
1 are: the expansion of the anterior part of the medullary plate and the projection of the fore-gut swelling 
beyond it; and the greater differentiation between the medullary plate and the posterior margin of the 
blastoderm. 
Fig. 6. Embryo 1.8 mm. in length. Normal plate series No. 6. H.E.C. 984. Dorsal view. X 8. 
The cephalic portion of the medullary plate is somewhat expanded and is raised a little more from 
: the blastoderm than is the posterior part. The greatest breadth of the cephalic portion is 0.7 mm. The 
j edges of the medullary plate are turned a little ventrally. A deep median furrow extends along the 
posterior five sixths of the plate. It is continuous posteriorly with the broad shallow rim notch. The 
posterior rim of the blastoderm is much thickened on either side of the rim notch for a distance of I mm. 
These thickened portions project backward a little over the yolk and represent the beginning of the tail 
swellings. The archenteron forms a broad tongue-like process which projects forward o.I mm. beyond the 
I anterior edge of the medullary plate. 
As compared with the preceding embryo of 1.6 mm., this embryo shows the following differences: 
the medullary plate is longer and more slender, and the anterior part (the cephalic plate) is expanded; 
the rim notch is deeper, and the tail swellings are differentiated from the rim of the blastoderm, 
Fig. 7. Embryo 2.0 mm. in length. Normal plate series No. 7. H.E.C. 1009. Dorsal view. X 8. 
F The cephalic portion of the medullary plate is broadly spatulate in outline and its edges are turned 
a little ventrally. Its length is 0.8 mm., its greatest breadth 0.6 mm. The boundary between the cephalic 
and trunk portions of the medullary plate is distinctly marked on the left side. A deep median furrow 
extends forward from the incisura neurenterica to the middle of the cephalic plate and ends at a low 
median ridge at this point. The cephalic plate is a little depressed on either side and in front of this 
ridge. This faint depression probably represents the beginning of the optic vesicles. The trunk part of 
j the medullary plate extends backward from the cephalic expansion as a raised band 0.3 mm. in breadth. 
‚ The medullary plate becomes indistinguishable from the tail swellings posteriorly. The lateral boundaries 
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