250 CLA VOL. [VoL. XIV. 
Cleavage and Blastoderm Formation. 
The cleavage in Anurida, and as far as observed in the rest 
of the Collembola and Thysanura, forms a striking exception 
to the method typical of the other Insecta. In Anurida the 
spherical egg has a cleavage that is slightly unequal but dis- 
tinctly holoblastic. The first cleavage plane cuts the ovum into 
two practically equal halves (Figs. 16,17). The second planes 
appear at slightly different places in the two halves and result in 
the arrangement of blastomeres so frequently found in annelids 
(Figs.18,19). Fig. 20 gives a different view of the 4-celled stage 
from that shown in Fig. 19, and illustrates the slightly unequal 
cleavage; this is almost a polar view and shows the shifting of 
the blastomeres. The second planes do not exactly halve the 
two parts of the egg. From this time on the planes appear 
regularly, the third being horizontal and the fourth more or less 
irregular, but in effect vertical, resulting in the 16-celled stage 
(Fig. 22). From this point on the details become confused by 
the rapid division and the difficulty of orientation. Holoblastic 
cleavage continues, however, up to the stage shown in Fig. 24, 
where a coarse morula stage has been reached. The cleavage 
planes are still distinct and the different cells stand up dis- 
tinctly on the surface. After this, however, a change takes 
place that is clearly visible on the surface; the cell outlines 
become indistinct and the blastomeres flattened ; until after 
another division the surface appears almost uniformly thickly 
scattered with white spots that represent the nuclei and sur- 
rounding protoplasm. 
A study of sections shows what has been taking place. 
Beginning with an egg slightly younger than that one shown 
in Fig. 16 the first cleavage spindle is distinctly seen (Fig. 28). 
The spindle does not differ in any way from the usual type. 
The centrospheres are represented by the darker haloes sur- 
rounding the ends of the spindle, and scattered through the 
protoplasm are small yolk granules in the process of transfor- 
mation into cytoplasm. Fig. 29 shows the reorganization of 
the nucleus and the amoeboid processes of the migrating 
masses of protoplasm. The number of chromosomes has not 
