162 



STUDY OF THE SEGMENTATION OF THE OVUM. 



globule, for its large size. It is usually spherical in fresh, oval in preserved 

 specimens, and measures in the living state from 22-28/4 in diameter. It has a 

 distinct cell-membrane, a protoplasm which resembles that of the ovum, and may 

 even contain granules of yolk. Soon after its separation from the ovum its 

 nucleus becomes well developed and membranate. Except, therefore, that the 

 number of chromosomes which enter into its formation is half the normal number, 

 we might say that it differs little from an ordinary cell. 



The Second Polar Globule. After the formation of the first polar globule 

 ovulation takes place, and during the next changes the ovum is situated in the 

 ampulla of the Fallopian tubes. In the mouse, unless the ovum is fertilized, it 

 forms no second polar globule, but instead undergoes autolysis either in the ovary 

 or in the uterine tube. The nucleus of the ovum does not enter into a condition 

 of repose, but at once transforms itself, as in other animals, into tl)e second polar 

 spindle. After the constricting off of the first polar globule, twelve half chromo- 

 somes (dyads) are left in the ovum. They are drawn into the equator of a new 

 spindle and ' split longitudinally. 



Pg.2 



Spz. 



FIG. 115. OVUM OF WHITE MOUSE, DIVIDING TO 



PRODUCE THE POLAR GLOBULE. 

 P.sp.2, Second polar spindle. Spz, Head of 



spermatozoon. X 500 diams. (After J . 



Sobotta.) 





FIG. 116. OVUM OF WHITE MOUSE, SHOWING THE 



METAPHASE OF THE DIVISION PRODUCING THE 



FIRST POLAR GLOBULE. 

 Pg. 2, Second polar globule, pi, Cell-plate. ?, 



Female pro-nucleus. X 1500 diams. (After 



J. Sobotta.) 



The second polar spindle is smaller than the first. It lies at right angles to 

 the axis of the ovum and quite close to the surface. It contains twelve thick 

 achromatic fibers, which do not unite at the poles with one another, but end par- 

 allel, so that the tip of the spindle is blunted. The chromosomes, when the mem- 

 brane first disappears, lie irregularly, but shortly after the formation of the spindle 

 they collect together to form an equatorial plate, somewhat as in the figure. 

 They are irregular and of uneven size, twelve in number, or possibly the number 

 may vary somewhat. The chromosomes then divide transversely, and the 

 halves move rapidly toward the ends of the spindle, which during this change 

 passes into the radial position (Fig. 115). The twenty-four univalent chromosomes 

 lengthen into filaments of various sizes, and by their form the second spindle can 



