39 J. T. CUNNINGHAM. 
The uncertainty with regard to the termination of the primi- 
tive blastopore indicated above occurs again here. If the 
primitive blastopore is confined entirely to the dorsal surface 
of the body, its posterior limit is marked by the neurenteric 
canal, whose position with regard to the blastoderm is indicated 
by the letter B in Diagram III. In this case the posterior 
part of the primitive streak in Sauropsida, (c) beyond the 
neurenteric canal, represents simply the coalescence of the 
non-inflected part of the edge of the blastoderm in Elasmo- 
branchii is the homologue of the yolk hernia in the latter, 
and shows that the Sauropsidan embryo in the course of evolu- 
tion has passed through a stage which is permanent in Elas- 
mobranchs. On this hypothesis, the fact, ascertained by 
Weldon (10), that in Lacerta the permanent anus is formed 
at a point in the posterior part of the primitive streak is of 
little importance; it would simply mean that the line of the 
first rupture of the blastoderm caused by the increased bulk 
of the yolk passed through the position of the permanent anus. 
If, on the contrary, the primitive ancestral blastopore really 
extends beyond the neurenteric canal, and posterior end of the 
body, on to the ventral surface, Weldon’s results give some 
support to the view that the permanent anus is derived from 
the primitive blastopore. In any case it is certain that the 
yolk in Sauropsida protrudes from a hernia in the ventral 
wall of the body which is not homologous with the yolk hernia 
in Elasmobranchs, and apparently not even continuous with 
that aperture. 
The homologous parts in the two cases are indicated by 
the letter c in Diagrams II and III, Pl. IV. Balfour con- 
cludes his remarks on the significance of the peculiar 
features of the Sauropsidan embryo with the following 
sentence: ‘The final enclosure of the yolk in the Sauro- 
psida takes place at the pole of the yolk-sac opposite the 
embryo, so that the blastopore is formed of three parts—(1) 
the neurenteric canal, (2) the primitive streak behind this, 
(3) the blastopore at the pole of the yolk-sac opposite the 
embryo.” 
ee 
