REPRODUCTION 129 



cium" was called by the two authors endomixis. In the case of P. 

 caudatum, they found endomixis occurs at intervals of about 60 

 days. Endomixis has since been reported in Spathidium spathula, 

 Euplotes longipes, Chilodonella uncinata, Didinium nasutum, Para- 

 mecium multimicronucleatum, Urostyla grandis, Paraclevelandia sim- 

 plex, etc. It appears to be another process of nuclear reorganization. 



As has already been stated, two types of nuclei: macronucleus 

 and micronucleus, occur in Euciliata and Suctoria. The macro- 

 nucleus is the center of the whole metabolic activity of the organism 

 and in the absence of this nucleus, the animal perishes. The waste 

 substances which become accumulated in the macronucleus through 

 its manifold activities, are apparently eliminated at the time of 

 division, as has been cited above in many species. On the other 

 hand, it is also probable that under certain circumstances, the macro- 

 nucleus becomes impregnated with waste materials which cannot be 

 eliminated through this process. Prior to and during conjugation 

 (p. 154) and autogamy (p. 161), the macronucleus becomes trans- 

 formed, in many species, into irregularly coiled thread-like sti;ucture 

 (Fig. 79) which undergoes segmentation into pieces and finally is 

 absorbed by the cytoplasm. New macronuclei are formed from some 

 of the division-products of micronuclei (synkarya) by probably in- 

 corporating the old macronuclear material. In most cases this sup- 

 position is not demonstrable. However, Kidder (1938) has shown in 

 the encysted Paraclevelandia simplex, an endocommensal of the 

 colon of certain wood-feeding roaches, this is actually the case; 

 namely, one of the divided micronuclei fuses directly with a part of 

 macronucleus (endomixis) to form a macronuclear anlage which 

 then develops into a macronucleus after passing through "ball-of- 

 yarn" stage similar to that which appears in an exconjugant of 

 Nyctotherus (Fig. 79). 



Since the macronucleus 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) observed that in 

 amicronucleate animals or when the micronuclei fail to give rise to 

 macronuclei, many (40 or more) pieces of the disintegrated old 

 macronucleus do not degenerate, but instead regenerate into new 

 macronuclei, which are segregated out to daughter individuals 

 formed at successive divisions, until one such regenerated macro- 

 nucleus is present in each individual. These macronuclei grow and 



