50 
The egg is now made up of one large micromere, a, at the 
nutritive pole, and at the formative pole six small, distinct 
spherules, ec, on one side of the polar globule, and one large 
one, b, on the other side ; this is flattened, in contact over a 
large area with the macromere, and is, without doubt, the 
second micomere of earlier stages. 
The history of the later stages shows that the single micro- 
mere, b, of this stage is anterior to the polar globule, while the 
group of six is posterior to it. The single macromere occu- 
pies what is to become the dorsal surface, and this figure is 
accordingly a view of the left side. For the sake of brevity, 
I shall in future use the terms dorsal and ventral, right and 
left, and anterior and posterior in describing the embryo, and 
it will be convenient to make this figure a reference mark. 
The ventral surface is here above, the dorsal below, the left 
side towards the observer, and the anterior end on the right 
side of the figure. 
Figure 23 is a view of the posterior surface of the same 
egg twenty-three minutes later. The micromeres which are 
posterior to the polar globule have now increased in number, 
and form a cap — the ectoderm — resting upon the macromere. 
The number of spherules, or ectoderm cells which form this 
layer, now increases rapidly by the division of large cells 
into smaller ones, and two couples which have been formed in 
this way are shown in the figure. The ectoderm is also in- 
creased by the separation of new spherules or micromeres 
from the macromere at the point where this touches the poste- 
rior border of the ectoderm. This portion of the ectoderm 
may therefore be called the growing edge. 
Figure 2 h is a view of the left side of the same egg five 
hours and fifteen minutes later. The anterior margin of the 
macromere is still separated from the polar globule by a sin- 
gle spherule, the second micromere, b , but posteriorly and at 
the sides the layer of ectoderm has grown considerably. At 
five points on the exposed side there are pairs of small cells, 
each of which has been formed by the division of a larger 
one. Figure 25 is another egg in nearly the same stage of 
development, but it has been copied here in order to show 
