THE OVUM. 



15 



Oolemma 



Achromatic spindle 

 Twin chromo-- 

 somes at 

 commence- 

 ment of 

 separation of 

 paternal and 

 maternal parts 



FIG. 15. SCHEMA OF MATUKATION OF OVUM IN 

 METAPHASE OF FIRST DIVISION. One pole of 

 the spindle projects into the first polar bud, 

 and the maternal and paternal parts of the 

 chromosomes are separating from each other. 



Oolemma 



Polar bud with chro- 

 mosomes, 



Achromatic >^> 



spindle /reXj 



lies in the region of the larger deutoplasmic granules by which its presence is 

 frequently obscured. 



The Mitochondria. These minute particles can be demonstrated by suitable 

 methods of fixation and staining. 



The Maturation of the Ovum. The process of maturation consists of two 

 mitotic divisions, of which the first is hetero- 

 typical, and results in the reduction of the 

 number of chromosomes, and the second is 

 homotypical. The phenomena of the two 

 divisions differ in some of their details from 

 those of ordinary cell divisions, therefore a 

 short account of them is necessary. 



In the prophase of the first maturation 

 division, the centrosome, the nucleolus, and 

 the nucleus vanish, and an achromatic spindle 

 appears at one pole of the oocyte, where it 

 lies, at first, parallel with the surface ; and 

 the chromosomes are gathered around its 

 equator. The number of the chromosomes 

 is only half the typical number, and they 

 are probably twin chromosomes (p. 11). 

 There are no centrosomes at the poles of the 

 spindle. After a short time the spindle 

 rotates until it lies at right angles to its 

 original position, and one pole, surrounded 

 by a small amount of the cytoplasm, forms 

 a projection, the first polar projection, on the 

 surface of the oocyte (Fig. 14). 



During the metaphase the twin chromo- 

 somes divide. In the anaphase the daughter 

 chromosomes travel to the opposite poles of 

 the spindle, and at the end of the anaphase 

 one-half of the daughter chromosomes lies 

 in the first polar projection and the other 

 half in the body of the oocyte (Fig. 16). 



In the telophase the first polar projection 

 is separated from the body of the oocyte and 

 oocyte I ceases to exist, being converted 

 into an oocyte of the second order, or oocyte 

 II, and the first polar body, each of which 

 contains half the typical number of chromo- 

 somes. 



The second maturation division occurs 

 without the intervention of a resting stage, 

 i.e. without the reappearance of a nucleus 

 in oocyte II. A new achromatic spindle 

 appears with the daughter chromosomes at 

 its equator ; it rotates, and one pole, sur- 

 rounded by a small amount of cytoplasm, 

 projects on the surface of the oocyte as the 

 second polar projection (Fig. 17). In the 

 metaphase the daughter chromosomes divide 

 homotypically into equal parts, and during 

 the anaphase the grand- daughter chromo- 

 somes move towards the poles of the spindle, 

 one-half entering the second polar projection and the other half remaining 

 in the body of the oocyte. During the telophase the second polar projection is 

 separated as the second polar body and the larger remaining part of the oocyte 

 II becomes the mature ovum (Figs. 1*7 and 18). 



Chromosomes 



which remain in 



oocyte II 



FIG. 16. SCHEMA OF MATURATION OF OVUM AT 

 END OF THE ANAPHASE OF THE FIRST DIVI- 

 SION. Two chromosomes (paternal or maternal) 

 lie in the first polar bud and two in the larger 

 part of the ovum which becomes oocyte II. 



Oolemma 

 First polar body - 



Second polar bud 



Chromosomes of 

 oocyte II 



Achromatic 

 spindle 



FIG. 17. SCHEMA OF MATURATION OF OVUM AT 

 THE COMMENCEMENT OF THE METAPHASE OF 

 THE SECOND DIVISION. 



