GENERAL EMBRYOLOGY 



1609 



After the second polar body has been extruded from the secondary 

 o6c\i:e the mature o\ami, or female pronucleus, passes from the 

 periphery to the centre, and there awaits the male pronucleus, 

 pro\aded fertilization is to take place. 



Nature of the Changes. — ^The germinal vesicle, or nucleus, ha\'ing 

 moved to the upper surface (animal pole) of the primary oocyte, 

 undergoes division, of the nature of karyokinesis, or mitosis, slightly 

 modified. Four phases have to be considered — namely, the anaphase, 

 metaphase, kataphase, and telophase. 



Anaphase. — ^The anaphase comprises four stages, (i) A spireme, 

 or skein, is formed from the chromatic reticulum of the germinal 

 vesicle. (2) This spireme breaks up into chromoses, which are dis- 



FiG. 664. — ^Maturation of Ovum (Wilson, by Permission of 



THE MaCMILLAN COMPANY, NEW YORK). 



A, Initial Phase B, Equatorial Plate C, Rotation of Spindle 



D, Formation of First Polar Body E, Dyads F, Division of First Polar Body 



G, Second Polar Body H, Three Polar Bodies 



posed as short rods. (Meanwhile the nuclear membrane of the 

 vesicle begins to disappear ; the germinal spot or nucleolus becomes 

 disintegrated ; and the karyol\-mph flows into the \-itellus, or cyto- 

 plasm.) (3) A spindle is formed, with a centrosome at each pole, 

 and the rod-like chromosomes lie parallel to the spindle-fibres. 

 (4) The chromosomes congregate at the equator of the spindle. 



At this stage the spindle lies parallel to the upper surface (animal 

 pole) of the ovum. It now, however, undergoes rotation, and 

 takes up a position at right angles to the animal pole of the ovum. 

 The superficial or peripheral pole of the spindle produces a slight 



