PLATE III. 
Figure 19. — The same egg two minutes and a half later, at 
the end of the third period of rest, viewed at right angles to* 
Figure 18. 
Figure 20. — The same egg thirteen minutes later, and in 
the same position as Figure 18. The spherule, c, of Figure 
19, has divided into two, and the second micromere, 5, has 
become prominent, so that there are five micromeres at the 
formative pole. 
Figure 21. — The same egg one minute later, and in the 
same position as Figure 19. 
Figure 22. — The same egg in the position of Figure 20, fif- 
teen minutes later than Figure 21, and in the fourth period 
of activity. There are now seven micromeres at the forma- 
tive pole, six on one side of the polar globules and one, the 
second micromere, b, on the other. 
Figure 23. — The same egg twenty-one minutes later, viewed 
from the side opposite the second micromere. The cells which 
have been formed by the division of the micromeres of the 
stage 19, now form a layer, the ectoderm, which rests, like a 
cap, on the macromere, a. 
Figure 24. — The same egg five hours and fifteen minutes 
later, in the same position as Figure 22, but not quite as 
much magnified. On one side the polar globule is still sepa- 
rated from the macromere, a, by a single spherule — the 
second micromere, b. Opposite this the growing edge, y, of 
the ectoderm is spreading still farther down over the macro- 
mere. At the point y, and at four other points, are pairs of 
small cells, which have evidently been formed by the division 
of the larger spherules. 
Figure 25.- Another egg at about the same stage. 
Figure 26. — The egg shown in Figure 24, fifty-live minutes 
later. The macromere, a, is almost covered by the ectoderm, 
and the second micromere, b , has divided into a number of 
spherules. At the growing edge, y, an ectoderm spherule is 
seen separating from the macromere. 
Figure 27. — A similar view of an egg twenty-seven hours 
after impregnation. The macromere is almost covered by the 
ectoderm, ec, and is not visible in a side surface- view. At y 
is an ectoderm spherule, which is separating from the macro- 
mere. 
Figure 28. — Optical section of the same egg ; ec , ectoderm ; 
en, macromere, divided into two spherules. So segmentation 
cavity can be seen in a normal egg at this or any of the pre- 
ceding stages. 
