550 A. C. WALTON 



is, however, no evidence of astral rays. The spindle seems to 

 be made up of many strands, or bmidles of fibers, each bundle 

 being associated \^-ith only one chromosome (fig. 27). Besides 

 these mantle fibers, there are others (fig. 28), which extend from 

 centrosome to centrosome ^^■ithout having any connection with 

 the chromosomes. 



The clu'omosomes are re-oriented on entering the metaphase 

 plate. The spindle fibers are attached to the sides of the chro- 

 mosomes and cause them to rotate so that in their new positions 

 they he with their long axes in the equatorial plane (fig. 28). 

 Hence the plane of the division must he along one of the longi- 

 tudinal splits of the di-tetrad. and the division must be longi- 

 tudinal. When seen in side view (fig. 27). the chromosomes 

 show a long axis parallel to the spindle fibers, but it is only of a 

 secondary nature. Figure 29 shows a much enlarged ^-iew of 

 the spmdle at this stage. The bands of fibers attached to each 

 chromosome are clearly distinguishable. The centrosomes show 

 as A'ery faintly staining granules at each pole of the spindle, 

 but without traces of an astral system. The chromosomes (fig. 

 29) all clearly show a decided polar elongation preparatory to 

 division, and in one, the halves are already puUed slightly apart. 



The nuclear elements remain in this condition until the entire 

 figure has migrated bochly from the center to the periphery of the 

 cell. Here it remams m an undivided condition until after the 

 entrance of the sperm and the formation of the fertihzation mem- 

 brane. This membrane is formed as a secretion of material by 

 the c}i:oplasm. incited by the chemical influence of the sperma- 

 tozoon, and is deposited just inside of the original ceU wall, 

 which remains distinctly separated from the apparently homo- 

 geneous fertilization membrane. 



The nuclear mass arrives at the periphery of the cell with the 

 long axis of the spindle parallel to a tangent to the nearest part 

 of the cell wall. Here it undergoes a rotation of ninety degrees 

 and comes to rest with one pole close against the cell membrane 

 (figs. 31 to 33 and 35). The chromosomes may or may not have 

 begun to pull in two bj- the time this final position of the spindle 

 is reached, but the action goes no further imless the egg has been 



