430 J. F. MeClendon. 
None of the three types of post-generation occurred in my 
experiments. The isolated blastomeres rounded up in a manner 
that might be compared to the first type of post-generation, but 
this regulation occurred before an embryo was formed, and might 
be distinguished as ‘‘immediate. ”’ 
It appears then that the egg at the time of the first cleavage is 
differentiated in the direction of the primary axis, and further- 
more in the direction of a plane passing through this axis and bi- 
secting the grey crescent, so that the egg may be-divided in this 
plane into two totipotent halves. The halves of the egg divided 
in any other plane may or may not be totipotent. The formation 
of a half embryo instead of a whole embryo from one blastomere 
of the two-cell stage is due to the presence and mechanical inter- 
ference of the other blastomere. 
In this connection may be mentioned the results of Endres, 
Herlitzka and Spemann on the development of the first two blasto- 
meres of urodeles when separated by constriction with a fine hair. 
If the constriction was slight, the hair merely marked the plane 
of the first cleavage, if it was deeper, double monsters occurred, 
and if it was complete, two perfect embryos resulte1. 
In from 66 per cent to 75 per cent of the eggs, the first cleavage 
plane became a frontal plane of the embryo, and in the remainder, 
it became the median plane. In the latter case complete constric- 
tion gave rise to two complete embryos, but in the former case, 
complete constriction resulted in one complete embryo developed 
from the prospectively dorsal half of the egg, whereas the pros- 
pectively ventral half did not develop beyond gastrulation. Thus 
it appears that the eggs of the urodeles possess the same organi- 
zation as the eggs of the anura, with the difference that the first 
cleavage plane is usually frontal in the former, and median 1 in the 
latter in relation to the embryonic axes. 
Accepted by The Wistar Institute of Anatomy and Biology, January 19, 1910. Printed June 21, 1910. 
