No. 3.] BASIS OF THE AMPHIBIAN EMBRYO. 477 
It is of interest to note that the location of this area of 
increased activity corresponds in position to a like area described 
by Lwoff in Amphioxus, Kolliker, Kionka, and others in the 
chick, and Assheton in the rabbit. 
In order to give a more graphic conception, I have introduced 
the accompanying diagram (Fig. 100). It represents the upper 
100 
hemisphere of an egg of Amblystoma in late cleavage. The 
dots represent pigment, which on one side of the upper hemi- 
sphere is much denser than on the other. This portion of the 
egg which is most deeply pigmented is likewise the portion in 
which cell activity is greatest. This activity is due to two dis- 
tinct areas of accelerated growth. The primary lies at the 
upper pole within the broken semicircular line c-d. The sec- 
ondary is less easily defined. It may be considered, however, 
as lying within the arcs e and f. These two areas define the 
position of the embryonic tract, and a line a—6 drawn through 
their centres represents the antero-posterior axis of the future 
embryo. 
The formation of the posterior portion of the embryo is due 
to a number of imperfectly comprehended forces, and it is con- 
sequently obvious that any explanation will be little more than 
a working hypothesis. 
There is no question but what the cells lying just above the 
dorsal lip of the blastopore are especially active. This neces- 
sitates either an infolding or a backward extension of the 
