REDUCTIOX /-V UNICELLULAR FORMS T.'J'J 



There can be no doubt that these polar spindles differ from the usual 

 type, and that they approach those recently described in the mitosis of 

 the higher plants, but it is doubtful whether the apparent absence of 

 asters and centrosomes is normal. In Ascaris, the first polar spindle 

 arising by a direct transformation of the germinal vesicle (Fig. 117) 

 has a barrel-shape, with no trace of asters. At the poles of the 

 spindle, however, are one or two deeply staining granules (Fig. 137)^ 

 which have been identified as centrosomes by Hacker ('94) and 

 Erlanger ('97, 4), but by Ftirst ('98) are regarded as central granules, 

 the whole spindle being conceived as an enlarged centrosome.^ For 

 the reasons stated at page 314, I believe the former to be the correct 

 interpretation.^ Spindles without centrosomes have been described in 

 the eggs of tunicates (Julin, Hill, Crampton), in AvipJiioxiis {'^^oWa), 

 in some species of copepods ( Hacker), and in some vertebrates {Dic- 

 myctyhis, Jordan ; mouse, Sobotta). In Ainphioxns (Sobotta) and 

 Triton (Carnoy and LeBrun) complete asters are not formed, but 

 fibrillas apparently corresponding to astral rays and converging to 

 the spindle-poles are found outside the limits of the spindle (Fig. 137). 

 In the guinea-pig, according to Montgomery ('98), centrosomes and 

 asters are present in the first polar spindle, but absent in the second. 

 The evidence is on the whole rather strong that the achromatic figure 

 in these cases approaches in form that seen in the higher plants ; 

 but it is an open question whether the appearances described may not 

 be a result of imperfect fixation. 



F. Reduction in Unicellul.a.r Forms 



Although the one-celled and other lower forms have not yet been 

 sutficiently investigated, we have already good ground for the conclu- 

 sion that a process analogous to the reduction of higher types regularly 

 recurs in them. In the conjugation of Infusoria, as already described 

 (p. 223), the original nucleus divides several times before union, and 

 only one of the resulting nuclei becomes the conjugating germ-nucleus, 

 while the others perish, like the polar bodies. The numerical corre- 

 spondence between the rejected nuclei or " corpuscules de rebut" has 

 already been pointed out (p. 227). Hertwig could not count the chro- 

 mosomes with absolute certainty, yet he states ('89) that m. Paravicr- 

 ciuvi camiatuni, during the final division, the number of spindle-fibres 

 and of the corresponding chromatic elements is but 4-6, while in the 



1(7/ p. 312. 



2 Sala ('94) and Fiirst have shown that occasionally the polar spindles of Ascaris are 

 provided with large typical asters, and thus resemble those of annelids or mollusks. Sala 

 believed this to be an effect of lowered temperature, but Fiirst's observations are unfavour- 

 able to this conclusion. 



