Phylum Ciliophora [ 225 



Earlier biologists supposed tliat the contractile vacuole is an excretory mechanism. 

 More probably, its function is purely hydrostatic, to rid the cell of the water which 

 is constantly entering by osmosis. Marine Ciliophora have no contractile vacuoles. 



In many members of the family Opalinoea each cell has many similar nuclei. These 

 divide, from time to time, by mitosis. Cell division takes place independently of 

 nuclear division, by transverse constriction, when a certain size has been reached. 



In the generaHty of Ciliophora, each cell has one or more nuclei of each of two 

 types, macronuclei, which are conspicuous, and micronuclei, discerned with difficulty. 



Cell division occurs by transverse constriction and is necessarily associated with 

 nuclear division. The macronucleus becomes elongate and divides by constriction 

 without any formation of chromosomes; in other words, amitotically. The micro- 

 nucleus also becomes elongate and divides by constriction. Early observers supposed 

 this process also to be amitotic. Actually, there appear within the intact nuclear 

 membrane a spindle and a definite number of chromosomes. Reichenow (editing 

 Doflein, 1927) compiled the following diploid counts: 



Stentor coeruleus 28 



Didinium nasutum 16 



Chilodon uncinatus 4 



C arche Slum poly pinum 16 



Turner (1930) found 8 in Euplotes Patella. Thus the chromosome numbers of 

 Cihophora appear usually to be small powers of 2. 



The chromosomes duly undergo division, the daughter chromosomes going to dif- 

 ferent ends of the nuclear cavity. The nucleus becomes greatly elongate and its mem- 

 brane presses in from the sides and cuts it in two. Turner observed in the axis of the 

 spindle of Euplotes a rather small endosome which becomes elongate and undergoes 

 constriction while the chromosomes are forming. 



Opalina has a sexual process in which the multinucleate cells divide into many 

 uninucleate gametes. These are sexually differentiated, larger and smaller; they 

 duly unite in pairs and the zygotes grow and become ordinary multinucleate 

 individuals. 



In the generality of Ciliophora, early observers discovered a sexual process in 

 which the cells, apparently undifferentiated, join in pairs but maintain their individ- 

 uality. The uniting cells become attached to each other in definite positions: in 

 Paramaecium, by their ventral or mouth-bearing surfaces; in Euplotes, by the left 

 halves of their broad ventral surfaces; in the ophryoscolecids and various other groups, 

 by their anterior ends. They remain attached, while continuing to swim, for several 

 hours, during which an exchange of nuclei takes place, and then resume their separate 

 life. Calkins (1926) was disposed, contrary to historical usage, to confine application 

 of the term conjugation to this exceptional form of syngamy. 



The nuclear details of conjugation were described by Maupas (1889) and Richard 

 Hertwig (1889), whose observations have repeatedly been confirmed. When a pair 

 have joined, their macronuclei divide several times; the ultimate fragments are 

 digested and disappear. The micronuclei also divide, concurrently in both conju- 

 gants, a fixed number of times, in Paramaecium three, in Euplotes four. These divi- 

 sions include a meiotic process. Most of the haploid nuclei produced are digested; 

 as a general rule, only one survives to undergo the final division, which is mitotic, 

 producing in each conjugant two genetically identical haploid nuclei. By this time 

 a cytoplasmic connection has been established between the conjugants. In Paramae- 

 cium, the spindles of the mitotic final nuclear divisions extend through this connection, 



