336 WHITMAN AND EYCLESHYMER. [Vor xr 
important point to be noticed in connection with this figure. 
The central blastomeres are represented as cut off from the 
yolk below. We feel confident that if the entire series be 
examined the cells will be found to be still continuous with the 
underlying yolk. 
The cleavage cavity in Amia has rather an interesting mode 
of origin. For the first appearance of this cavity is in the form 
of central vacuoles, which are present as early as the first 
cleavage groove. The cleavage grooves, in many cases at 
least, expand into broad fissure-like cavities as they approach 
the centre of the egg (Cuts 3, 5, 15), where they become con- 
tinuous with the earlier cavity or cavities. As the cleavage 
runs on these spaces enlarge, flow together, and thus often give 
rise to quite a spacious cleavage cavity, such as shown in Cuts 
10, 17, 19. In some cases the cavity is so much reduced that 
it becomes inconspicuous. 
Fourth Cleavage. — The fourth cleavage marks an interesting 
epoch, as it is the first to take a direct part in the formation of 
small cells at the animal pole. For the complete delimitation 
of the cells another cleavage is necessary, namely, a horizontal 
cleavage which cannot be seen from the surface and which 
affects only the eight cells bounded superficially by the fourth 
cleavage grooves, or by what we may call the circular groove. 
The latter usually appears midway between the pole and the 
margin of the calotte. It represents, strictly speaking, eight 
distinct grooves, one for each of the eight primary segments, 
but these usually run together in such a way as to form a con- 
tinuous groove, encircling the pole and bounding a polar field 
which may have a circular form (Figs. 6, 25, 26) or a more 
elongated oval form (Figs. 16, 18). In one egg (Fig. 6) we 
have traced the order and direction of these grooves, noting 
also the time and place of origin as indicated in the figure. 
There are many variations in this cleavage. It often hap- 
pens that the eight constituent grooves run at various angles 
and are discontinuous, so that the polar field is bounded by a 
more or less irregular or zigzag line (Figs. 16, 18) ; sometimes 
one or more of these grooves take a meridional direction, as 
that shown in the upper part of Fig. 18. 

