FERTILIZATION 



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two functional pronuclei are derived from different spindles. In all cases 

 in which only one nucleus is involved, and possibly also in those where 

 two or more are involved, the two pronuclei must be genetically identical 

 if the third division is equational. In Uroleptus (Calkins, 1919), two or 

 three nuclei divide, but the two pronuclei are always sister nuclei. As 

 this occurs in both members of the pair, the exconjugants should theo- 

 retically be genetically identical. This appears to be the significant feature 

 in the third maturation division. 



Figure 151. Euplotes patella. A, B, C, third maturation division, in which the four 

 haploid chromosomes in A split longitudinally and the halves slip past each other 

 in B, and four go to each pole in C; D, fertilization nucleus in which the $ and $ 

 pronuclei have just joined, but their chromosome groups have not yet mingled. 



In Euplotes patella (Turner, 1930) , two nuclei enter the third division. 

 In each nucleus (Fig. 151) the chromatin reticulum condenses into four 

 strands of chromatin granules, which condense into four compact sausage- 

 shaped chromosomes lying lengthwise of the spindle. The chromosomes 

 split longitudinally, and the halves slip past each other as they migrate 

 to separate poles in this equational division. The chromosomes are all 

 lying in the same axis, so that as the chromosomes slip past each other 

 in the early anaphase, they appear end to end, and the figure might 

 easily be misinterpreted as a transverse division of chromosomes. It is 

 possible that the descriptions of the transverse division of chromosomes 

 in the third division, given by Enriques (1908), Calkins (1919, 1930), 

 MacDougall (1925), and others were based on some such artifact. Cal- 

 kins points out that if each chromosome represents one gene, the method 

 of division is of no consequence. This interpretation would doubtless 

 serve for Paramecium, which has a large number of chromosomes, but it 

 is hardly conceivable that Chilodonella would have only two pairs of 

 genes. It seems more probable that the apparent transverse division of 

 some chromosomes in mitosis is due to our inability to demonstrate by 



