254 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
and it will be readily seen that there is not the slightest order in 
their arrangement, nor is there any regularity in the way they 
advance. Figure 3 represents a portion of the same blastoderm on 
a larger scale. The peripheral protoplasmic layer has the same 
thickness as in the last egg mentioned, except at those places where 
nuclei have entered it. Here its thickness is increased by the 
diameter of the nucleus, which on the average is 12.5 /i. 
Figure 4 represents a section of an egg in which the formation 
of the blastoderm is still farther advanced. The nuclei begin to 
crowd one another at the surface of the egg, and, not finding room 
for themselves in the narrow, peripheral, protoplasmic layer, they 
begin to project outward, and thus form protuberances on the 
outer surface of the egg, giving a section the wavy outline shown 
in the figure. This protoplasmic layer has increased somewhat in 
thickness (probably as a result of the consumption of yolk granules 
by the rapidly dividing nuclei), and is now 7.5/x thick between the 
nuclei. As will be seen in Figure 5, which shows a much enlarged 
view of a portion of this blastoderm, the nuclei have nearly the 
same diameter as those shown in Figures 2 and 3, and are prolifer¬ 
ating much more rapidly. The protoplasmic paths are not so 
numerous as in the egg last described, and will become less 
frequent from now on, showing that the yolk granules move 
together again, thus obliterating the paths. The gradual thickening 
of the peripheral protoplasmic layer also probably goes on at the 
expense of these paths. 
These processes all continue. The segmentation nuclei at the 
surface increase very rapidly in number, and begin to decrease in 
size as they become more and more numerous, and the protuber¬ 
ances which they occupy begin to crowd one another (PI. 1, Fig. 6). 
These do not, however, show a tendency to merge with one another. 
Each protuberance becomes rather more clearly defined and dis¬ 
tinct, and its sides, from forming acute angles with the plane of the 
former surface of the egg, as shown in Figure 4, finally come to 
form right angles with it (Fig. 6). The surface of the egg then 
presents a curious appearance, being like the pavement of a street 
the blocks of which are separated by deep narrow spaces. The 
average diameter of the nuclei in the egg rejwesented in Figure 6 
is 5 /x. The peripheral protoplasmic layer has also increased in 
thickness, and now measures 15 /x at points between the nuclei. 
It now constitutes a blastema, such as is present in Musca before 
segmentation begins. 
