326 BIOLOGY. [BOOK iv. 



to invoke one day in support of their mysteries the most impor- 

 tant and the most improbable, is not rare in the animal kingdom. 

 Many examples thereof can be cited, of which the most celebrated 

 is the parthenogenesis of the pucerons, signalised first of all by 

 Bonnet. 



But for the most part the spontaneous evolution of the 

 ovulum does not go further than the emergence of the polar 

 globules. If then the fecundation does not intervene, the ovulum 

 withers and dissolves. In the contrary case, it undergoes the 

 important phenomenon of fractionment, that is to say, a series 

 of fissiparous divisions, of bipartitions. Fissiparity has certainly 



FIG. 49. 



First stadium of the evolution of a mammifer, " segmentation of the ovum," multiplica- 

 tion of the cells by reiterated scissions : A, the ovum divides by a first fissure into two 

 cells ; U, the two cells divide into four cells ; C, these last divide into eight cells ; D, the 

 segmentation, indefinitely reiterated, has produced a spherical mass of numerous cells. 



been the first process of reproduction of the rudimentary 

 organisms, and it still exists in the state of transitory phase, of 

 organic tradition, in sexuate generation. The phenomenon is one 

 of the simplest. After the disappearance of the vesicle, the 

 retraction of the vitellus, the emergence of the polar globules, the 

 vitellus narrows in the middle, and is thus severed into two granu- 

 lous masses, of which each is divided in its turn into two halves, 

 and so on in succession, till the moment when the whole vitelline 

 mass is transformed into a heap of globules, spherical, granulous, 

 destitute at first of enveloping membranes and of nuclei, then 



