30 K. MITSUKURI; 



all points of its circumference over the yolk. At Balfour's stage A, 

 however, the first trace of the enib3'o appears as a slight thickening 

 at one j)oint of the circumference of the blastoderm. This point is 

 usually regarded as the hind end of the l)lastoderm. This is not 

 quite correct, for it really becomes the front end of the future embryo. 

 After the appearance of the embryonic rim the blastoderm still con- 

 tinues to sj)read over the yolk by a uniform growth of all parts of its 

 circumference, l)ut 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 

 einbrycjnic rim travels awav 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 tlie embi-yonic rim at any given moment 

 of its srowth — is sliuhtlv elevated from the rest of the blastoderm, 

 and traversed by an inconspicuous longitudinal median groove. This 

 raised part of the blastoderm soon becomes marked otf by two ridges, 

 wdiich 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 tlie 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 

 swellirigs ' of lx\LrouE. 'J'his elevated part of the l)lastoderm is the 

 medullary ])Iate, and tl>e shallow groove traversing it marks the line 

 of growth of the indentation abt;)ve referred to. These points are all 

 illustrated by my fig. 1,* which represents the em1)ryo at a stage 

 wdiere the indented rim has grown back a considerable distance from 

 the point of its first appearance. * * * * 



* See the two figures copied from Sedgwick on PI. VII. 



