342 WHITMAN AND EYCLESHYMER. (Mor. ATh 
EXPLANATION OF CUTS 13 TO Ig. 
Cur 13.— Oblique section of an egg in the stage of the fourth cleavage, just 
before the fifth cleavage. The plane of the section is represented by the dotted 
line 13-13 in Cut 7. The section shows on one side that the fourth cleavage has 
not yet cut off the central cells. 
Curt 14.— Oblique section of the same stage, passing along the line 14-14 of 
Cut 6. The section shows the plane of elongation of the nuclei in the marginal 
segments which are soon to be divided by a set of verticals, forming a part of the 
fifth cleavage. 
Cur 15.— Oblique section along the line 15-15 of Cut 7. The section 
shows that the circular groove (IV) becomes continuous below with the vacuolar 
spaces. 
Cuts 16 and 18.— Vertical sections of the calotte from different eggs in the 
stage of fourth cleavage. The sections show the vertical elongation of the nuclei 
of the central cells preparatory to the horizontal cleavage which is to divide 
them. 
Cut 17.— Oblique section of an egg in a stage a little earlier than that 
shown in Cut 19. The section passes in the plane indicated by the line 17-17 
in Cut 19. The section shows an exceptionally large cleavage cavity. It also 
shows numerous yolk nuclei lying at the inner ends of the large yolk segments. 
CuT 19.— Vertical section of a typical blastula. The cleavage cavity in this 
egg is also exceptionally large. Some of the large yolk segments may be seen 
dividing at their inner ends, the cells thus derived being continually added to the 
calotte. 
