560 ADAM SEDGWICK. 
really becomes the front end of the future embryo. After the 
appearance of the embryonic rim the blastoderm still con- 
tinues to spread over the yolk by a uniform growth of all 
parts of its circumference, but in the centre of the embryonic 
rim a slight indentation appears. This indentation shares in 
the uniform growth of the blastoderm edge, and advances over 
the yolk equally with the rest of the embryonic rim and 
general edge of the blastoderm. As the embryonic rim 
travels away from the point of its first appearance, the surface 
of the blastoderm so formed—that is to say, the surface of the 
part of the blastoderm extending between the point of first 
appearance of the embryonic rim and the embryonic rim at 
any given moment of its growth—is slightly elevated from the 
rest of the blastoderm, and traversed by an inconspicuous 
longitudinal median groove. This raised part of the blasto- 
derm soon becomes marked off by two ridges, which in front, 
i.e. at the point which marks the site of the first appearance 
of the embryonic rim, are continuous with one another, while 
behind they are continuous with the parts of the embryonic 
rim which bound the indentation. These portions of the 
embryonic rim are more markedly swollen than the rest, and 
form the “ tail swellings’ of Balfour. This elevated part of 
the blastoderm is the medullary plate, and the shallow groove 
traversing it marks the line of growth of the indentation 
above referred to. These points are all illustrated by my 
fig. 1, which represents the embryo at a stage where the 
indented embryonic rim has grown back a considerable dis- 
tance from the point of its first appearance. Various stages 
in the process may be seen in Balfour’s figures! of Stages B, 
C, D, and in Schwarz’s figs. 1 and 2. The indentation of the 
embryonic rim is always placed at the hind end of the groove 
which marks the centre of the medullary plate. This groove 
is a transitory structure, and soon disappears ; its importance 
consists in the fact that it indicates the line of growth of the 
indentation of the embryonic rim. (It is conterminous in 
1 ¢ Monograph of Development of Elasmobranch Fishes,’ pl. vii; pl. vill 
of the Memorial Edition, 
