228 



FERTILIZATION OF THE OVUM 



the mitotic figure. At either pole of the spindle is a centrosome, 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 

 rapid multiplication of the nucleus followed, by a corresponding divi- 

 sion of the cell-body to form " spores," which remain for a time 

 closely aggregated before their liberation. The resemblance of this 



Fig. 113. Conjugation of bpirogyia. [OVERTON.] 



A. Union of the conjugating cells (S. communis}. B. The typical, though not invariable, 

 mode of fusion in S. Weberi ; the chromatophore 'of the "female" cell breaks in the middle, 

 while that of the " male " cell passes into the interval. C. The resulting zygospore filled with 

 pyrenoids, before union of the nuclei. D. Zygospore after fusion of the nuclei and formation 

 of the membrane. 



process to the fertilization and subsequent cleavage of the ovum is 

 particularly striking. 



The conjugation of unicellular plants shows some interesting 

 features. Here the conjugating cells completely fuse to form a 

 "zygospore" (Figs. 113, 140), which as a rule becomes surrounded 

 by a thick membrane, and, unlike the animal conjugate, may long 

 remain in a quiescent state before division. Not only do the nuclei 



