562 ADAM SEDGWICK. 
its edge, which is part of the general edge of the blastoderm, 
the ectoderm is continuous with the endoderm which forms 
the under side of the tongue. A good idea of the appearance 
of a transverse section through this tongue is given by fig. 
1 4, pl. x,! of the ‘ Elasmobranch Fishes’ (Mem. Ed.). The 
hinder end of the tongue is of course notched, and the notch 
is continued forwards along the line of the groove above men- 
tioned as occupying the centre of the medullary plate, as a 
slit which actually completely perforates the blastoderm, so as 
to lead into the space between the endoderm of the tongue 
and the yolk. This is shown clearly in fig. 3, and at a later 
stage in fig. 4. Whether this slit is due to a bilobed back- 
ward growth of the notched portion of the embryonic rim, 
the growth at the middle point, 7. e. at the bottom of the notch, 
ceasing—in other words, to an emphasising of the notch already 
present—or whether it arises as a secondary perforation of the 
medullary plate and endoderm along the line of the groove 
before mentioned, I am unable to say; but I think it is due to 
the former. 
While these changes have been taking place—and I must 
now refer back to fig. 2—the sides of the projecting tongue 
become bent ventralwards and towards each other until they 
meet or nearly meet in the ventral middle line. Now two 
important structural results, which should be noted and under- 
stood, follow from this bending: (i) the two angles formed by 
the junction of the edge of the blastoderm in the embryonic 
region with the edge of the blastoderm in the non-embry- 
onic region—the angles, one of which is marked a in fig. 2, 
become closely approximated ventrally beneath the embryo; 
and (11) a space is enclosed on the ventral side of the embryo, 
which space is lined by endoderm, and opens ventrally to the 
exterior through a slit formed by the contact of the ventrally 
bent edges of the tongue, and dorsally into the neural canal 
by the slit in the medullary plate. This space? is the hind 
' Old edition, pl. ix, fig. 14. 
? A section of the tongue in this stage in front of the neurenteric slit is 
shown in Schwarz’s fig. 16. 
