SEGMENTATION OF THE VITELLUS. 



897 



to their passage. Shortly after fecundation, the germinal vesicle disappears ; but this 

 occurs in ova that have not been fecundated. Soon after ovulation, also, the vitellus 

 gradually withdraws itself from certain portions of the vitelline membrane, or becomes 

 deformed, and then often rotates upon itself; a phenomenon which has long been observed 

 in the ova of some of the lowest orders of animals and of rabbits. The deformation 

 and gyration of the vitellus, however, have been observed in ova before fecundation and 

 have nothing to do with the process of development. They are of the class of move- 

 ments called amoeboid. 



After the penetration of spermatozoids and their union with the vitellus, at least in 

 many of the lowest orders of animals, the appearance of the vitellus undergoes a remark- 

 able change, by which ova that are about to pass through the first processes of develop- 

 ment may be readily distinguished from those which have not been fecundated. This 

 change consists in an enlargement of the granules and their more complete separation 

 from the clear substance of the vitellus. The granules then refract light more strongly 

 than before, so that the fecundated ova are distinctly brighter than the others. This is 

 the first appearance that is distinctive of fecundation. 



Polar Globule. The next process observed in the ovum is the separation from the 

 vitellus of a comparatively clear, rounded mass, called by Robin the polar globule. This 

 body has been observed before by various anatomists and described under different 

 names. The exact mode of its formation has been studied by Robin in some of the lower 



FIG. 288. Formation of the polar globules in the ova of the Nephelis octoculata. (Bobin.) 



orders of animals. We shall describe briefly this process as it was demonstrated to ns by 

 Robin, in 1861, the description being taken from notes made at that time : 



Five hours after the entrance of the spermatozoids, we see a little elevation at one 

 point in the vitellus. This is the beginning of the polar globule. It increases in size 

 gradually, and becomes constricted at its base, until it is attached to the vitellus by a 

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