NOTES ON ELASMOBRANCH DEVELOPMENT. 235 



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 3. 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 Pishes,' pi. vii ; pi. viii 

 of the Memorial Edition. 



