REPRODUCTION 153 



Since the macro-nucleus originates in a micronucleus, it must con- 

 tain all structures which characterize the micronucleus. Why then 

 does it not divide mitotically as does the micronucleus? During 

 conjugation or autogamy in a ciliate, the macronucleus degenerates, 

 disintegrates and finally becomes absorbed in the cytoplasm. In 

 Paramecium aurelia, Sonneborn (1940, 1942, 1947) (Fig. 57) ob- 

 served that when the animal in conjugation is exposed to 38°C. 

 from the time of the synkaryon-formation until before the second 

 postzygotic nuclear division (a-c), the development of the two newly 

 formed macronuclei is retarded and do not divide as usual with the 

 result that one of the individuals formed by the second postzygotic 

 division receives the newly formed macronucleus, while the other 

 lacks this (c). In the latter, however, division continues, during 

 which some of the original 20-40 pieces of the old macronucleus that 

 have been present in the cytoplasm segregate in approximately equal 

 number at each division (d, e) until there is only one in the animal 

 (/). Thereafter the macronucleus divides at each division (g). Sonne- 

 born found this "macronuclear regeneration" in the varieties 1 and 

 4, but considered that it occurs in all stocks. Thus the macronucleus 

 in this ciliate appears to be a compound structure with its 20-40 

 component parts, each containing all that is needed for development 

 into a complete macronucleus. From these observations, Sonneborn 

 concludes that the macronucleus in P. aurelia appears to undergo 

 amitosis, since it is a compound nucleus composed of many "sub- 

 nuclei" and since at fission all that is necessary to bring about 

 genetically equivalent functional macronuclei is to segregate these 

 multiple subnuclei into two random groups. 



While the macronuclear division usually follows the micronuclear 

 division, it takes place in the absence of the latter as seen in amicro- 

 nucleate individuals of ciliates which possess normally a micronu- 

 cleus. Amicronucleate ciliates have been found to occur naturally or 

 produced experimentally in the following species: Didinium nasutum 

 (Thon, 1905; Patten, 1921), Oxytricha hymenostoma (Dawson, 1919), 

 O.fallax, Urostyla grandis (Woodruff , 1921), Paramecium caudatum 

 (Landis, 1920; Woodruff, 1921), etc. Amicronucleate Oxytricha f alia x 

 which were kept under observation by Reynolds (1932) for 29 

 months, showed the same course of regeneration as the normal indi- 

 viduals. Beers (1946b) saw no difference in vegetative activity be- 

 tween amicronucleate and normal individuals of Tillina magna. In 

 Euplotes patella, amicronucleates arise from "double" form (p. 229) 

 with a single micronucleus, and Kimball (1941a) found that the 

 mioronucleus is not essential for continued life in at least some 



