306 WILSON. (VoL. III. 
of individual cells (No. 53), and probably does not affect the 
essential character or the end-result of the cleavage. The 
following description applies to L. fetzdus. 
III. CLEAVAGE. 
1. The fertile ova are slightly ovoid in shape and measure 
about 0.125 mm. by 0.112 mm. They are surrounded by a 
delicate vitelline membrane which is at first closely applied to 
the vitellus, but afterwards enlarges so as to become widely sepa- 
rated from it. The vitellus is laden with small, clear yolk-spheres 
and granules, which only partially obscure its transparency and 
usually do not prevent a clear view of the interior of the 
embryo after suitable treatment with reagents. Several polar 
cells are formed, probably by the division of two primary polar 
cells directly separated from the ovum. They usually occur in 
two groups of three or four each (Figs. 1 and 2) which lie in 
the plane of the first cleavage, but appear afterwards to shift 
their position so that no constant relation between them and 
the embryo can be recognized. At the first cleavage the 
vitellus divides into two unequal parts, the plane of division 
passing through the polar cells (Fig. 2). The second cleavage 
divides the smaller cell into two nearly equal parts, and (as I 
believe) separates a third small cell from the larger of the first 
two (Figs. 3-5). No constant difference in the granulation 
of these four cells can be made out. In the ensuing resting 
stage a distinct cavity appears between the’ three smaller and 
the one larger cell, but this cavity disappears afterwards, and 
cannot be identified with the true blastoccel. I have occasionally 
seen apparently normal ova in which the four cells of this stage 
are nearly equal in size, and others in which only three cells 
were present. With the third cleavage normal variation cer- 
tainly begins. Asa rule, the largest of the four cells shown in 
Figs. 3-5 divides into. two equal parts, and the three smaller 
cells lie in a group above the angle between them (Fig. 6). In 
some cases, however, only two small cells are present. In the 
next stage, as a rule, the number of small cells is increased 
to four (Fig. 7), probably by division of one of the smaller 
cells of the preceding stage, after which-each of the large cells 
divides into two unequal parts (Figs. 8-14), the smaller of 
