544 BIOLOGY OF THE PROTOZOA 



plate in meiosis would be eqiii\'alent to eqiiational division. The 

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

 Chilodon, Uroleptiis, Didinium, etc.), and the anomalous condition 

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

 equational and one reducing division at meiosis, as against one 

 equational and one reducing division in Metazoa. With different 

 forms furthermore reduction may occur either in the first division 

 as in Oxytricha, or in the second division as in the majority of cases 

 on record. Dehorne (1920) escapes the difficulty by finding in 

 Paramecmm caudatwn that there are no chromosomes at all, the 

 single, much-looped filament of chromatin diA'iding transversely at 

 each division. 



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 would 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 fallax cited 

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

 which has been advanced in any ciliate (with the exception possibly 

 of Styloiiychia puHiulaia, see above), the functional nucleus would 

 have 12 dumb-bell-sha])ed 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 dinnb-bell should separate then the normal diploid number 

 would be restored with two sets of homologous chromosomes and 

 the 48 chromosomes would be formed h\ the further division of 

 the 24. 



Still further difficulties are added by the merotomy experiments 

 with conjugating UroJeptus mohilis. A pair in conjugation at the 

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

 Fig. 220. The angular apex thus cut off and one of the arms, are 

 fixed and stained to determine the stage of maturation. The other 

 arm is cultivated. Since other pronuclei usually degenerate, it is 

 evident that only one pronucleus is present in the piece cultivated, 

 and this one contains the haploid number of chromosomes. The 

 possibility remains open, however, that this jironucleus may unite 

 with a sister pronucleus formed by sister nuclei, and which do not 

 degenerate. In this case it would be parthenogenesis of the third 

 type above. Wlien such cutting experiments are successful the 

 resultant organisms regenerate perfectly and undergo typical life 

 histories and each individual has the normal number of chromo- 

 somes. 



