228 



FERTII.IZATIOy OF THE OVUM 



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

 origin of which remains undetermined. 



it is an interesting fact that in Noctilnca, 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 oi Spirogyta. [OvF.RTON.] 

 A. Union of the conjugating cells {S. comnmnis). B. The typical, though not invariable, 

 mode of fusion in 5. 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 membr^e. 



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 



