i6io A MANUAL OF ANATOMY 



projection of the vitellus, or cytoplasm, which is known as the 

 polar bud. 



Metaphase. — The chromoses, being now close to the equator of 

 the spindle, are broken up into daughter-chromoses. 



Kataphase. — The daughter-chromosomes now separate, half of 

 them travelling along [metakinesis) the spindle-fibres to the upper 

 pole of the spindle, where a centrosome awaits them, and half remain 

 in the primary oocyte. 



Telophase. — Cell-division now takes place in the primary oocyte. 

 The upper pole of the spindle, with its contained daughter-chromo- 

 somes, is cut off and extruded as the first polar body, after the 

 entrance of a spermatozoon. This takes with it the vitellus, or 

 cytoplasm, forming the polar bud, as well as some filaments from 

 the spindle, and those which connect the daughter-chromosomes. 



This concludes the first cell-division, which affects the primary 

 oocyte. 



The interrupted spindle is now completed, and a new centrosome 

 is formed. The spindle takes up a position at first parallel to the 

 animal pole of the secondary oocyte, and then rotates to become 

 at right angles to it. The daughter-chromosomes congregate at 

 the equator of the spindle, and undergo splitting. Half of them 

 pass to the upper pole of the spindle, and half remain in the 

 secondary oocyte. The subsequent phases are as in the first cell- 

 division. A second polar body is extruded, carrying with it some 

 of the vitellus, and the remainder of the secondary oocyte, much the 

 larger part, forms the mature ovum, or female pronucleus, which 

 now takes up a central position. 



Fertilization of the Ovum. 



Fertilization is otherwise spoken of as impregnation, or fecundation. 

 It :onsists in the conjugation or fusion of the male pronucleus, or 

 head of a spermatozoon, with the female pronucleus, or mature 

 ovum, and it constitutes the commencement of the development of 

 a new individual to propagate the species. As a general rule, con- 

 jugation takes place in the outer part of the Fallopian tube, or 

 oviduct, into which spermatozoa have made their way through the 

 vagina and uterus by the lashing movement of their tails. 



When the spermatozoa come into contact with the mature ovum 

 one of them as a rule passes through the zona pellucida, or radiata, 

 into the yolk. At the point of entrance the yolk forms a conical 

 protuberance, called the receptive, or entrance cone. As the sperma- 

 tozoon passes through this cone it parts with its tail, the surrounding 

 vitellus becoming disposed in a radiating manner. Meanwhile a 

 delicate membrane is formed round the yolk, called the vitelline 

 membrane, which prevents the entrance into the yolk of other 

 spermatozoa as a rule. 



The head, or nucleus, of the sperm.atozoon now constitutes the 

 male pronucleus, or sperm-nucleus, and the middle piece contaiis 



