1 84 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



is formed (Fig. 84, A) having at each of its poles a minute, deeply-staining, 

 particle, the centrosome. The two tetrads lie at the equator of the spindle 

 and each becomes split apart into two halves — dyads — which gradually 



Fig. 84. 



Maturation of the gametes in A scaris — male on the left and female on the right (after Brauer and 

 Boveri). A, Commencing first meiotic division — the two tetrads are seen at the equator of the spindle 

 — a centrosome is present at each pole of the spindle in the male but not in the female. B, The first 

 meiotic division nearly complete, the two daughter cells — each containing two dyad chromosomes — 

 are equal in size in the male but very unequal in the female. C, Second meiotic division. In the male 

 each of the two cells resulting from the preceding division is dividing, the upper cell being shown 

 at a later stage than the lower. Each cell contains two monads and a centrosome. In the female 

 the daughter cells are again very unequal {" Egg " and second polar body). I, first polar body ; 

 II, second polar body. (In B — left hand fig. — the centrosome at each pole of the spindle has pre- 

 cociously divided into two, in preparation for the second mitosis.) 



recede from one another towards the poles of the spindle, possibly owing 

 to the contraction of spindle fibres attached to them (Fig. 84, B). As 

 the dyads move apart a constriction appears round the equator of the 

 cell which gradually deepens until the cell-body is completely divided 



