324 BIOLOGY. [Bock iv. 



vesicle is the least variable part of the ovum. It can be double, 

 and then gemelliparity seems constantly to result therefrom. 

 This is what takes place at least in certain species, especially in 

 the Vort&c balticus, in which this duplicity of the_ germinative 

 vesicle is the rule.^ 



The cellular content or vitellus is likewise called yellow, very 

 improperly, however, for its colour is very variable. In effect, it 

 is, according to species, white, yellow, red, brown, green, violet, 

 and so on, and is usually composed of a liquid more or less 

 viscous, holding in suspension granules, and often fatty globules. 



The vitelline or enveloping membrane is also exceedingly 

 diverse. Often it is a transparent, anhysted membrane ; some- 

 times it is a simple albuminous layer. In certain species it is 

 covered with designs, with grooves, with reliefs. At other 

 times, as happens in the ovulum of fishes, it is piel'ced with 

 holes, with micropylse, which facilitate the impregnation of the 

 ovulum by the spermatozoaries.^ 



We must then consider the nucleus of the ovulum as a germ 

 englobed in a mass purely nutritive, which is the vitellus. 



The ovulum of viviparous animals differs very little as to 

 volume, whatever the size of the animals may be. That of the 

 oviparous animals is much larger, is sometimes indeed very 

 voluminous ; but the increase bears on the nutritive portion of 

 the ovulum, the vitellus, which in birds acquires an enormous 

 development. 



What distinguishes the ovulum .froni every other cell, even 

 before fecxmdation, is the rapidity of its evolution. There is 

 a whole series of phases, of modifications, through which it 

 passes in a very short space of time, without even undergoing 

 the fecundating impulsion. Soon, in the whole animal series, 

 the ovulum loses its first differentiation ; the nucleus and the 

 nucleole (vesicle and germinative spot) disappear, and seem to 

 melt into the vitellus. The ovulum is then nothing more than 



' Leydig, TraiU d'Histologie, p. 620. 

 ' Leydig, loc. eit., pp. 616, 617. 



