102 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
8. Firta CuEavaGe. THIRTY-TWO CELLS. 
All of the sixteen cells of the previous stage are involved in the fifth 
cleavage, but the primary mesoblast cell (d**) and the yolk-entoblast 
(d°*) are greatly retarded in division (Plate 5, Figs. 44-46). The four- 
teen cells of the blastoderm divide about synchronously, but occasion- 
ally some of the anterior cells slightly precede in the cleavage (Plate 5, 
Figs. 44, 45 ; Plate 6, Fig. 47; Plate 12, Figs. 112, 113). The nuclear 
spindles for this cleavage are arranged perpendicularly to those of the 
preceding cleavage, with the exception of those in the three mes-ecto- 
blast cells (a*?, 0°-?, c®?), which touch the yolk-cell at the blastopore 
(Fig. 46). The spindles in the cells a®:? and c*? are always somewhat 
oblique to those of the preceding cleavage (compare Figs. 40, 45, 46). 
They appear to be arranged more or less at right angles to the lines 
along which the greatest pressure would be exerted by the contiguous 
cells of the blastoderm (see Figs. 45, 46), and the arrangement therefore 
seems to be in accord with the principle that spindles tend to become 
arranged in the line of least resistance. 
The spindle in the median cell 0°:? is sometimes placed almost longi- 
tudinally (Figure 113), in which case the resulting cells (4°, b*-4, Fig. 
46) are arranged as in Figures 48, 52 and 116. Sometimes the spin- 
dle in 0°? is almost transverse (Fig. 112) and the resulting arrange- 
ment of the daughter cells is shown in Figure 51. Many intermediate 
oblique positions of spindle and cleavage plane have been noted. This, 
too, is apparently a case of adjustment to least resistance. In the next 
stage these two cells (b°*, 6°*) become so shifted in position that they 
lie one to the right and the other to the left of the sagittal plane, but 
usually one is more or less in front of its companion. In the sixty-two- 
cell stage their derivatives always form the anterior boundary of the 
blastopore, although in the thirty-two-cell stage one of the cells (5%) 
may not be in immediate contact with the yolk-entoblast, a condition 
shown in Figures 48 and 52. 
In Figure 70 (Plate 8) it is noticeable that the cleavage planes which 
separate the mes-ectoblasts a&*, and c* from their sister cells (a%*, ¢°*) 
are markedly oblique, so that the latter overlap the former. Attention 
is here called to the tendency of cells around the blastopore to divide in 
this manner, for in the succeeding stage there is a similar oblique divis- 
ion of a3 and c®.8, and the inner derivatives are overgrown by the outer 
overlapping cells. 
