334 WHITMAN AND EYCLESHYMER. [VoL. XII. 
Cut 3 represents a vertical section of an egg at a stage just 
preceding the fourth cleavage. Its position is shown by the 
dotted line in Diagr. B. The section runs parallel with the 
second meridional, and cuts through two of the cells delimited 
by the third verticals (III). The first meridional has at this 
time cut entirely through the calotte and deeply into the under- 
lying yolk. The third set of grooves have likewise extended 
far into the yolk. These grooves usually pass in vertical 
planes ; at times, however, they are more or less inclined ; we 
have sections in which one or more of them pass so obliquely 
that the cells are deeply cut, but in no case have we found 
any of them entirely severed from the underlying yolk. Occa- 
sionally one finds eggs in which one of these grooves (III®) 
occupies the position shown in Diagr. D, Cut 5. From sur- 
face study one might be in doubt whether this groove is verti- 
cal or horizontal. Serial sections of three such eggs show that 
the groove is vertical, and that the segment in each case is 
continuous with the underlying yolk. 
In Cut 5 a horizontal section of such an egg is shown which 
passes just beneath the calotte. All the third cleavage grooves 
(III) except one (III) are arranged radially, alternating with 
the two primary grooves. It is noteworthy that all these 
grooves widen into fissures which have a common vacuolar 
centre, giving rise to a star-shaped figure. 
Cut 4 exhibits a vertical section of the same stage, but at 
right angles to the plane of Cut 3. The grooves on the surface 
of the egg are shown in Diagr. C, and the position of the | 
section is indicated by the dotted line. The section is taken 
near the margin of the calotte. Sections nearer the centre of 
the egg show that the first and second grooves have reached a 
much greater depth, and have become continuous with cavities 
similar to those shown in Cut 3. 
Dean (No. 3, p. 426) states that “the segmentation cavity 
takes its definite origin at this stage; in the region of 
the animal pole the blastomeres are separated from the 
underlying yolk—the germ disc by a narrow fissure, which 
has been found to arise in the cleavage planes of the 
animal pole.” 

