102 



Cellular Structure and Activity 



spindle is commonly proportional to the vol- 

 ume of the nvicleus from which it originates. 

 This is especially clear during cleavage of 

 many organisms (Conklin, '12) where nu- 

 clei and spindles are large in early divisions 

 and get smaller in late ones. In Pediculopsis 

 the spindle volume decreases from the first 

 to the tenth cleavage about 200 fold (Cooper, 

 '39). Other good illustrations are the large 

 and small spermatocjrtes of Arvelius (Schra- 

 der, '47). Where more than one nucleus is 

 present within a cell, each forms its own 

 spindle. During early cleavage sperm and 



to all spindles is the bipolar organization. 

 This bipolarity is independent of centriole 

 and aster as demonstrated by the cases where 

 centrioles are naturally inactive (for in- 

 stance in oogenesis of Ascaris) or experi- 

 mentally inhibited (Bataillon and Tchou 

 Su, '30). 



In the living cell the spindle is a gelatin- 

 ous semi-solid body that can be moved about 

 in the cell or even dissected out with the 

 micromanipulator (Chambers, '24; Carlson, 

 '52). Cytoplasmic granules never penetrate 

 the spindle (which differentiates the true 



Fig. 17. The birefringence of the spindle. A, Pollen mother cell of Liliwn lonpiflorum (phot. Inoue). B, 

 Amphiaster and spindle isolated from the blastomere of a sea urchin (phot. Inoue. see Mazia and Dan, '52) . 



egg chromosomes often remain separate and 

 each group forms its own spindle (gonom- 

 ery). Such independent spindles may fuse 

 into one or remain separate through ana- 

 phase (Hughes-Schrader, '24). The spindle 

 may become organized inside the nuclear 

 membrane or only after the membrane has 

 dissolved. In some cells the membrane does 

 not disappear until anaphase or even persists 

 throughout mitosis (intranuclear spindles; 

 cf. Drosophila cleavage. Fig. 14). The form 

 of the spindle is very variable; it may be 

 shaped like a disc, a barrel or a spindle, or 

 may be flared at either end, or it may be 

 asymmetrical (Fig. 16). Where a centriole is 

 present the spindle is usually pointed at the 

 ends and terminates at the centriole unless 

 the asters are very large compared to the 

 spindle (Fig. 16B). In animal cells where 

 the centriole is inactive the spindle is usu- 

 ally barrel-shaped (Fig. 16/1). But common 



mitotic spindle from the amphiaster or cen- 

 tral spindle. Fig. 15) and in living cells 

 they can be seen bouncing off the spindle 

 (Ris, '43). At metaphase the spindle is firmly 

 anchored to the asters and the whole spindle 

 apparatus (achromatic figure) can be moved 

 about or even isolated from the cell (Mazia 

 and Dan, '52 (Fig. 175). After fixation the 

 spindle generally has a fibrous structure 

 (continuous fibers). In the living spindle 

 this is only rarely visible (Cooper, '41), but 

 Lewis ('23) has shown long ago that a 

 change in pH can make it appear reversibly. 

 Recently Inoue ('52) has demonstrated con- 

 tinuous fibers in living spindles with an im- 

 proved polarizing microscope. Even where 

 spindle fibers are not visible in life they can 

 no longer be regarded as artifacts; they are 

 an expression of the basic organization of 

 the spindle. In the polarizing microscope the 

 spindle shows a positive birefringence with 



