i68 



FERTILIZATIOX OF THE OVUM 



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

 in the maturation and fertilization of the ^^g. 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 

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

 bodies (p. 175), 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 



A 



B 



C 



Fig. 84. — Conjugation of 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. 



is the fact that the gcnn-niiclci unite zchen 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. 82, G ), and this gives good reason 

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

 tributed 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 

 Noctiluca, which has been studied with some care by Cienkowsky 

 and Ishikawa (Fig. 84). Here the conjugating animals completely 

 fuse, but the nuclei are merely apposed and give rise each to one- 

 half of the mitotic figure. At either pole of the spindle is a cen- 

 trosome, the origin of which remains undetermined. 



It is an interesting fact that in Noctiluca, in the Gregarines, and 

 probably in some other Protozoa, conjugation is followed by a very 



