44 



PROTOZOA. 



being rapid, there will always be a considerable number of 

 descendants of the same exconjugate in juxtaposition. 



Maupas also found that exhaustion of the food supply, which 

 usually determines encystrnent, causes conjugation among individuals 

 of mixed origin and of suitable age. 



The morphological changes, our knowledge of which we owe 

 mainly to Balbiani, Biitschli, and particularly Maupas, appear to 

 consist of the breaking-up and disappearance of the macronucleus, 

 the particles of which in *ome cases, at any rate are excreted 



FIG. 37. Conjugation of Pammeecium caudatum after R. Hertwig. A" macronucleus ; n micro- 

 nucleus ; o mouth. In a the micronucleus is forming a spindle ; l> illustrates the second 

 division of the micronucleus, resulting in the formation of four micronuclei in each animal ; 

 of these, those marked 2, 3, 4, 6, 7, 8 will atrophy, while 1 and 5 will persist and divide ; c, 

 the micronuclei 2, 3, 4, 6, 7, 8 are disappearing, while 1 and 5 are dividing into two, one of 

 which, 1m and 5m, migrate respectively through the protoplasmic connection into the other 

 conjugating individual, while the other, Iw and 5w, remains in its own individual. 



through the anus; and in the increase by successive bipartitions 

 of the micronucleus. Of the micronuclei so produced, all abort 

 except two, one of which is migratory and the other stationary ; the 

 migratory nucleus passes into the other gamete, and fuses with its 

 stationary nucleus to form the zygote-nucleus. The two gametes, 

 each with a zygote-nucleus, now separate, and the zygote-nucleus 

 gives rise by successive bipartitions to the nuclear apparatus of the 

 exconjugate. The details differ in different species. As an example 

 of the process we may take the simple case of Colpidium colpoda, 

 which possesses one macro- and one micronucleus. Soon after the 

 onset of conjugation, which lasts some hours, the micronucleus of 

 each gamete undergoes two successive bipartitions, and so gives rise 



