30 STRUCTURE AND LIFE-HISTORY OF A SPONGE. 
of change as the parent ovum, the nucleus of each disappears, 
and is replaced by two fresh nuclei, each is then furrowed by a 
constriction, and eventually divided into two cells. The plane of 
division is still perpendicular, but at right angles to the previous 
one. The four cells thus resulting resemble the parent cells in 
being broad and flat below, and narrower towards the top ; they 
are likewise flattened against each other, but rounded off along 
their inner perpendicular edges, so as to leave a small cavity or 
canal in the axis of the group. This canal is the beginning of 
what we shall know hereafter as the cleavage cavity. Division 
again takes place, and still in a perpendicular direction ; each of 
the four cells is divided into two from top to bottom, and thus 
eight cells arise, which form a ring, surrounding the cleavage 
cavity, and narrowing from the base upwards. After these three 
perpendicular divisions, which have given us first two, then four, 
and finally eight, cells, a fourth one occurs, which is horizontal, 
and so at right angles to all the preceding ones. As the tops of 
the cells, previous to this division, were, as stated before, smaller 
than the bottoms, so it follows that the eight upper cells above the 
median plane of division are smaller than the eight lower cells 
beneath it. The embryo consists now of i6 cells in two rows, an 
upper or apical row, and a lower or basal row, of eight cells each, 
surrounding the cleavage cavity, which opens by a wide lumen 
below, and a much narrower aperture above. The embryo ex- 
changes now its plano-convex or cake-like for a cushion-like, or 
biconvex, form. Two more divisions, also in a horizontal direction, 
now succeed, severing each ring into two, and thus producing a 
four ringed form ; one ring is apical, one basal, and the two 
between may be called equatorial. The cells of the equatorial 
rings are again divided, and this time vertically, in a meridional 
direction^ we might say ; in this way each equatorial row comes to 
contain sixteen cells. The embryo now is a cushion-like sac, the 
wall being composed of a single layer of similar cells, forty-eight 
