542 BIOLOGY OF THE PROTOZOA 



tion, the third division as absent in aurelia but present in caudatum. 

 If reduction occurs with the first two divisions the four products 

 in endomixis are equivalent to haploid nuclei so far as the chromo- 

 somes are concerned, and correspond, therefore, to the first t'y^^ 

 of parthenogenesis above. But they are likewise equivalent to the 

 fertilization nucleus and develop with the diploid number of chromo- 

 somes. This number, unfortunately is too large in Paramecium 

 to permit of accurate counting, while in ciliates with a small number 

 of chromosomes, endomixis takes place during encystment where 

 cytological details have not been made out in any case. Fermor 

 (1912) indeed, described the union of the two macronuclei and of the 

 two micronuclei in Stylonychia jmstidata during encystment but the 

 account of the phenomenon is incomplete and on its face implies 

 the fusion of diploid nuclei. This is so improbable from the chromo- 

 some standpoint that the result cannot be accepted without 

 confirmation. 



As indicated above (p. 527) the difficulty over haploid and diploid 

 chromosome number reaches an extreme in connection with the 

 third division of the ciliate nucleus. If reduction in number occurs 

 during the first two meiotic divisions then the pronuclei are formed 

 by a third division of an haploid number of chromosomes. If 

 this division is transverse as appears to be the case with Para- 

 mecium, this third division might also be a reducing division, and 

 the amphinucleus coming from the union of such nuclei would 

 be haploid. If the third division however, is equational the pro- 

 nuclei would still have the haploid number and their fusion would 

 result in a diploid amphinucleus. The latter appears to be the 

 correct solution. Gregory (1923) for example describes 24 dumb- 

 bell-shaped chromosomes in the nuclear plate of the first meiotic 

 division of O.vytricha fallax. This number is reduced to 12 dumb- 

 bell-shape chromosomes with this first division and each dumb- 

 bell di^•ides longitudinally. The equational halves are separated at 

 the second division and 12 dumb-bells form the equatorial plate 

 of the third division (Fig. 227). The two halves of the dumb-bell 

 are finally separated with this third division, 12 single chromosomes 

 passing to each pole. The pronuclei thus have 12 single chromo- 

 somes and the amphinucleus formed by their union has 24. The 

 interpretation here depends upon the origin of the 24 chromosomes 

 of the first division. 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 nvnnber of chromosomes were 48 this would be synapsis in the 

 usual sense. The "reduced number, however, is 12 and onl}' 24 

 chromosomes make up the amphinucleus. If the granules are 

 homologous and in ])airs, and if like unites with like to form the 

 (hnnb-bells, then dixision of the 24 chromosomes of the first nuclear 



