RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 31 
The true state of the case seems to me to be expressed thus :— 
The real blastopore in Sauropsida, representing the blastopore 
of the ancestral gastrula, comprises the anterior part of the 
primitive streak in front of the neurenteric canal, the neuren- 
teric canal itself, and perhaps a portion of the posterior part of 
the primitive streak. The yolk hernia in Elasmobranchs is 
represented in Sauropsida by the posterior part of the primi- 
tive streak. The edge of the blastoderm in Sauropsida is a 
new yolk-hernia peculiar to this type, and closing towards the 
centre instead of from end to end like the hernia in Elasmo- 
branchs. Thus the inflected part of the edge of the blasto- 
derm in Elasmobranchs is homologous with the whole edge in 
other Ichthyopsida, and no part of the edge of the blastoderm 
in Sauropsida is homologous with the edge of the blastoderm 
in Ichthyopsida. 
The relations now described are represented graphically by 
the diagrams figs. 24—30. The diagrams all represent 
sections through the plane of symmetry of the developing 
ovum, but the superficial boundary of the blastoderm is 
represented by the dotted line ed, and the internal limit 
of the dorsal hypoblast in figs. 24, 25, 26, and 29 by the 
dotted line ss. Figs. 27 and 28 are simplified ideal re- 
presentations of two stages in the development of the ancestral 
vertebrate gastrula. The stage in fig. 27 corresponds to the 
stage in the Teleostean represented by fig. 24, while similarly 
fig. 28 corresponds to fig. 26. The part of the hypoblast in 
fig 27, between the lines o and P, corresponds to the nucleated 
yolk (yolk-mass plus superficial layer of periblast) in figs. 24, 
25, and 26, and to the nucleated yolk in figs. 29 and 30. The 
neurochord is purposely omitted in all the figures, because it 
is only, as it were, accidentally connected with the relations of 
the blastopore. The notochord (no.) and mesoblast (me.) are 
both represented by continuous black shading, The presence 
of mesoblast between hypoblast and epiblast in figs. 25 and 26 is 
purely hypothetical ; it is probably not developed till after the 
complete closure of the blastopore. The letter z marks the an- 
terior end of the embryo, the letter y the point at which con- 
