PHENOMENA ACCOMPANYING FERTILIZATION 321 



depends upon the origin of the 24 or 48 chromosomes of the first 

 division. In Oxytricha the meiotic process begins with a spireme 

 which fragments into granules, approximately 48 in number. 

 Association of these granules, 2 by 2, results in 24 dumb-bells. If 

 the number of chromosomes were 48 this would be synapsis in the 

 usual sense. The reduced number, however, is 12 and only 24 

 chromosomes make up the amphinucleus. If the granules are 

 homologous and in pairs, and if like unites with like to form the 

 dumb-bells, then division of the 24 chromosomes of the first nuclear 

 plate in meiosis would be equivalent to equational division. The 

 latter interpretation satisfies the conditions in other ciliates (e. g., 

 Chilodon, Uroleptus, Dddinium, etc.), and the anomalous condition 

 in ciliates generally may be cleared up by the assumption of two 

 equational and one reducing division per chromosome at meiosis, as 

 against one equational and one reducing division in Metazoa. With 

 all forms, furthermore, reduction occurs during the first two meiotic 

 divisions. The difficulties, however, cannot be cleared up by a priori, 

 reasoning in attempts to homologize protozoan and metazoan 

 meiosis. In Uroleptus halseyi all three meiotic divisions during 

 conjugation are transverse divisions and these chromosomes find 

 their place in the theory of the gene only on the assumption that 

 each chromosome represents one gene and one gene only (Calkins, 

 1930). 



A further difficulty arises with parthenogenesis. Woodruff and 

 Erdmann regard the first two divisions of the nucleus at endomixis 

 as equivalent to the first two divisions in conjugation. If this is 

 true the chromosomes are presumably reduced in number by either 

 the first or the second division and the reorganization nucleus woidd 

 be haploid from which the normal number of chromosomes in endo- 

 mictic animals would have to be reestablished by division of each 

 of the chromosomes present. In the case of Oxytricha faUax cited 

 above, barring fusion of nuclei during endomixis, no evidence for 

 which has been advanced in any filiate, the functional nucleus would 

 have 12 dumb-bell-shaped chromosomes. If the chromosomes 

 remain double a race of haploid individuals would be formed. At 

 the next endomictic period these would again be halved, and so on. 

 This, however, is unbelievable. If on the other hand the parts 

 of the dumb-bell should separate, then the normal diploid number 

 would be restored with two sets of homologous chromosomes and 

 the 48 chromosomes w r ould be formed by the further division of 

 the 24, and this would be intelligible on the above assumption of 

 a single gene per chromosome. 



Still further difficulties are added by the merotomy experiments 



with conjugating Uroleptus mobilis. A pair in conjugation at the 



period of pronuclei interchange is cut across the angle as shown in 



Fig. 155. The angular apex thus cut off and one of the arms are 



21 



