340 WHITMAN AND EVCLESHYMER. [VoL. XII. 
takes effect only in the four central cells of the 16-cell stage. 
The marginal cells are divided by verticals which correspond to 
what happens in the eight yolk segments of the Amia egg. 
Variations in this cleavage are numerous. It often happens 
that one or more of the central cells are cut by grooves passing 
parallel with the circular. Again, some of the grooves, instead 
of taking a horizontal or a circular form, may pass vertically, 
as indicated both by surface views and the elongation of the 
dividing nuclei. Whether the grooves cutting the central cells 
shown in Fig. 7 are to be interpreted as variations, or whether 
they represent a part of the sixth cleavage, we are unable to 
say. In this egg the central grooves appeared shortly after 
the appearance of the grooves dividing the marginal cells, with 
which they soon became continuous. 
Sections of this stage are shown in Cuts 9, 14, 16, and 18. 
In Cut 14 the plane of elongation of the nuclei in the large 
marginal cells confirms what has been described from surface 
views. In Cuts 16 and 18 the elongation of the nuclei (V) 
lying within the circular groove (IV) shows that the next 
division of these cells will take place in a horizontal plane. 
While this is undoubtedly the usual plane of division, there are 
exceptions. In each of the sections shown in Cuts 14 and 15 
one cell of the eight delimited by the fourth cleavage shows its 
nucleus elongated in such a direction that the resulting cleav- 
age will be circular. In other sections the elongation of the, 
nuclei indicates that vertical cleavages may also replace the 
horizontal. Cut 9 represents a vertical section of an egg in 
the stage shown in Fig. 8. The horizontal cleavage has evi- 
dently taken place, and in addition to this another set of verti- 
cals have appeared. In this section it was impossible to trace 
the cleavage grooves through the yolk. 
Later Cleavage. — The next cleavage, which might be called 
the sixth, is shown in Figs. 8 and 28, and consists in a general 
way of two sets of circular furrows, one of which appears 
between the first circular and the upper pole, the other between 
the first circular and the margin of the calotte. In addition to 
these, new verticals have arisen in some of the marginal 
segments. 

