CONJUGATION IN UNICELLULAR FORMS 



227 



one-eighth of the original micronucleus (Fig. iii). Each germ- 

 nucleus divides into two, as usual, but one of the products of each 

 degenerates, and the two remaining pronuclei conjugate to form a 

 cleavage-nucleus. 



The facts just described show a very close parallel to those observed 

 in the maturation and fertilization of the o.^^. In both cases there 

 is a union of two similar nuclei to form a cleavage-nucleus or its 

 equivalent, equally derived from both gametes, and this is the pro- 

 genitor of all the nuclei of the daughter-cells arising by subsequent 

 divisions. In both cases, moreover (if we confine the comparison 

 to the egg), the original nucleus does not conjugate with its fellow 

 until it has by division produced a number of other nuclei all but 

 one of which degenerate. Maupas does not hesitate to compare 



B 



Fig. 112. — Conjugation o{ Noctiluca. [Ishikawa.] 

 A. Union of the gametes, apposition of the nuclei. B. Complete fusion of the gametes. 

 Above and below the apposed nuclei are the centrosomes. C. Cleavage-spindle, consisting of 

 two separate halves. 



these degenerating nuclei or "corpuscules dc rebut" with the polar 

 bodies (p. 181), and it is a remarkable coincidence that their number; 

 like that of the polar bodies, is often three, though this is not always 

 the case. 



A remarkable peculiarity in the conjugation of the Infusoria 

 is the fact that the germ-nuclei laiite ivheji in the form of spindles 

 or mitotic figures. These spindles consist of achromatic fibres, or 

 " archoplasm," and chromosomes, but no asters or undoubted cen- 

 trosomes have been thus far seen in them. During union the 

 spindles join side by side (Fig. i io, G), and this gives good reason 

 to believe that the chromatin of the two gametes is equally distrib- 

 uted to the daughter-nuclei as in Metazoa. In the conjugation of 

 some other Protozoa the nuclei unite while in the resting state ; but 

 very little is known of the process save in the cystoflagellate Nocti- 

 luca, which has been studied with some care by Cienkowsky and 

 Ishikawa (Fig. 112). Here the conjugating animals completely fuse, 

 but the nuclei are merely apposed and give rise each to one-half of 



