GENERAL EMBRYOLOGY 



i6xx 



a centrosome, called the spermo-centre. The male pronucleus 

 advances towards the centre of the ovum, near which, up till now, 

 the mature o\-um. or female pronucleus, is l>-ing quiescent, being 

 destitute of its original centrosome, which has disappeared. As the 

 male pronucleus, along with its centrosome, advances, the centro- 

 some leading the way, the female pronucleus shows receptive signs, 

 and moves slightly to meet the approaching \'isitor. The two 

 pronuclei then come into very near contact, not far from the centre 

 of the ovum, but they do not as yet fuse. The male centrosome, or 

 spermo-centre, now divides, and two centrosomes are formed, one 



First Stage Second Stage Third Stage 



Fig. 665. — Fertilization of Ovum. 



A, Entnnce of Spermatozoon B, Extrusion of Polar Bodies C, Male Pronaclcos 



D, Compound Nucleus (Male and Female Pronucleus) 



E, Female Pronucleus (Ovum) F, Female Pronucleus 



of which passes to the distal side of the female pronucleus. Con- 

 jugation or fusion of the two pronuclei now takes place, and the 

 mixed nucleus thus produced is called the segmentation, or cleavage- 

 nucleus. 



This completes the stage of fertilization. A chromatic spireme, 

 or skein, chromosome, and a spindle are subsequently formed within 

 the segmentation-nucleus, and the segmentation-stage is entered 

 upon — that is to say, cell-division of the nature of mitosis, or karyo- 

 kinesis. takes place. 



Certain authorities maintain that the ovum, after parting with the second 

 polar body, retains a centrosome, known as the ovo-centre. The male pro- 

 nucleus, as stated, brings with it a centrosome or spermo-centre. Each ovo- 

 centre and spermo-centre divides into two, and each division of the ovo- 

 centre joins a di\-ision of the spermo-centre. When, therefore, fusion of the 

 two pronuclei has been effected, the resultant segmentation-nucleus has two 

 mixed centrosomes (male and female) , one on either side, or at each pole. 



Segmentation of the Ovum. 



Segmentation consists in the division of the fertilized ovum into 

 a mass of cells. 



After the mature ovum, or female pronucleus, has been fertilized 

 by fusion with the male pronucleus, mitotic or karyokinetic cell- 

 division commences, and the o\'um is ultimately transformed into 



