102 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



8. FIFTH CLEAVAGE. THIRTY-TWO CELLS. 



All of the sixteen cells of the previous stage are involved in the fifth 

 cleavage, but the primary mesoblast cell (d 6 - 2 ) and the yolk-eutoblast 

 (d 5 - 1 ) 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 5 - 2 , 5 ' 2 , e 5 ' 2 ), which touch the yolk-cell at the blastopore 

 (Fig. 46). The spindles in the cells a 5 - 2 and c 5 - 2 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 6 5 - 2 is sometimes placed almost longi- 

 tudinally (Figure 113), in which case the resulting cells (5 6 - 3 , 6 ' 4 , Fig. 

 46) are arranged as in Figures 48, 52 and 116. Sometimes the spin- 

 dle in & 5 * 2 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 (6 6 * 8 , b 6A ) 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 6 - 8 ) 

 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 6 ' 8 , and c 6 * 8 from their sister cells (a 6 - 4 , c 6 - 4 ) 

 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 a 6 ' 8 and c 6 . 8 , and the inner derivatives are overgrown by the outer 

 overlapping cells. 



