DR. E. B. WILSON ON THE DEVELOPMENT OF RE NILE A. 
731 
activity continued for a long time, until the spheres had become very small and the 
embryo had begun to elongate. It is clearly shown in the series of figures from 
1 to 18, of which the first fourteen are from one individual (time, 115 minutes), the 
last four from another specimen (time, 32 minutes). The intervals of time between 
the successive visible cleavages were somewhat irregular, as shown in the following 
statement 
Between figures 
5 5 
55 
2 and 
5 . 
9 minutes. 
5 „ 
7 . 
.... 45 „ 
7 „ 
9 . 
.... 29 
9 „ 
11 . 
.... 12 „ 
11 „ 
13 . 
.... 23 „ 
13 „ 
15 . 
Not observed. 
15 „ 
17 . 
27 minutes. 
In the eggs of many animals the periods follow one another with great uniformity, 
and the irregularity in the present case is therefore somewhat unusual. It depends 
perhaps on the fact that the embryo is solid, and that during the whole segmentation 
the cleavages take place not only in planes at right angles to the surface, but also in 
planes parallel to it. The latter cleavages would not be visible externally, but might 
retard the surface cleavages at certain periods. This is apparently the true explana¬ 
tion of the long delay of forty-five minutes between figs. 5 and 7 ; for, as we shall see 
in the following section, the delamination, by means of which the layers are separated, 
takes place at this period when the embryo consists of sixteen spheres. 
2. The mode of segmentation which has been described occurred with slight varia¬ 
tions in rather less than one-third of all the eggs studied. In the most usual case, 
however, the eight-sphere stage is entirely passed over, and the egg divides at once 
into sixteen spheres at the first cleavage. 
This mode of cleavage, illustrated by figs. 30 to 37, is, except in the first stage, 
quite like the cleavage into eight spheres. The egg is at first perfectly spherical, then 
becomes irregular in form, with a wavy outline, and at length falls at once into sixteen 
spheres (fig. 33), which are, as a rule, of equal size. Though very distinct at first, they 
soon flatten together, and the egg passes into a resting stage (fig. 35), which continues 
for ten to twenty minutes. This quiescent period, though only slightly marked in the 
specimen figured, is sometimes very pronounced, so that the embryo may be nearly or 
quite indistinguishable from the unsegmented egg. The subsequent development is 
very regular, and is like the first case. 
As noted above, the spheres are usually of equal size. It is, however, a common 
occurrence for the segmentation to be more or less unequal, as shown in figs. 38 to 44. 
In this case the embryo presents externally the appearance of an epibolic gastrula, 
consisting of macromeres and micromeres. In the eight-sphere stage, also, embryos were 
