49 
sharply separated than at the preceding stage, and a vesicle 
has made its appearance in each. After two minutes and a 
half, 19, these three micromeres have flattened down against 
each other and against the compound mass, so that the egg is 
once more nearly spherical. This stage may be called the 
third period of reet. 
This figure and the one before it are from the same egg, as 
indeed are all the figures on this plate except 25, 27 and 28 ; 
but after Figure 18 had been sketched, the egg rotated a little, 
and Figure 19 is a view at right angles to all the preceding 
ones. 
The vesicles are seen to be a little larger than they were a 
few minutes before, but I did not succeed in seeing them dis- 
appear at this stage. 
The egg which was figured remained in the condition shown 
in Figure 19 for thirteen minutes, and during this time it 
rotated back again into the position which it had occupied at 
first. 
At the end of thirteen minutes the three micromeres <?, d , d , 
again became conspicuous; the compound mass, a and b, 
elongated, and a surface depression separated the portion b 
from the portion a , and the first micromere, a, quickly divided 
into two, as shown in Figure 20. 
By moving the cover-glass, I managed to rotate the eggs a 
little, and to get a sketch, Figure 21, in the same position 
as Figure 19. Figure 21 is one minute later than Figure 20, 
and it wall be seen that the second micromere, b , and two of 
the others, have already begun to flatten down and to pass 
into the resting condition. 
From this time on I w T as not able to keep the egg under 
constant observation, but examined it at short intervals. A 
well marked resting period follows the stage shown in Figure 
21, but as it presents no new features, it was not figured. 
Figure 22 is fifteen minutes later, and shows the egg at the 
fourth period of activity. There are now two more micro- 
meres, which appear to be formed by the tw r o, c c , Figure 20, 
which were produced during the third period of activity by 
the division of the first, Figure 14, c. 
4 
