
No. 427.] THE EGG OF BUFO LENTIGINOSUS. 535 
the blastopore appears below the equator of the egg at the 
edge of the black cells (Fig. 1), and that the blastopore rim 
extends at a uniform rate in the form of a crescent around 
the lower hemisphere, until finally the yolk plug is formed 
(Figs. 2-4). Meanwhile, the dorsal lip of the blastopore has 
moved over the yolk, and seven hours after its first appearance 
has reached the center of the white hemisphere (Fig. 6). The 
closure of the blastopore lips takes place almost uniformly from 
all sides as a rule (Fig. 7), although occasionally the blastopore 
is somewhat oval when it is in the stage of Fig. 8. At or just 
before the stage of Fig. 8 a rotation of the egg around its 
horizontal axis takes place, thus bringing the dorsal lip of the 
blastopore back to its original position. 
In its movement over the lower hemisphere the dorsal lip of 
the blastopore seems to pass through about 140° of the surface 
of the egg. There is apparently a slight variation in this respect 
in different eggs. This measurement is somewhat greater than 
that given by Morgan (16) and by Wilson (27) for the move- 
ment of the dorsal lip of the blastopore in the frog egg, and 
differs considerably from the measurements which have been 
given by other investigators. Roux (21) and Pflüger (19) esti- 
mate that the dorsal lip moves about 170°, while Kopsch (11) 
considers the movement to be but 75°; and this figure is further 
reduced by Assheton, who states that much of the so-called 
movement of the dorsal lip is only apparent and that the real 
movement is only from 60° to 70°. 
THE FORMATION OF THE “GERM LAYERS" AND THE 
NOTOCHORD. 
Observations regarding the origin of the endoderm, meso- 
derm, and notochord in different amphibian eggs are so 
conflicting that apparently the most plausible interpretation 
of them is to assume that there is absolutely no uniformity 
in the manner of formation of these structures even in 
species that are considered to be very nearly related to 
each other. Such an assumption, however, can scarcely be 
the correct one. 
