CHORION 



Fig. 116. Fertilization and maturation of the egg in the urochordate, Styela (Cynthia) 

 partita. (After Conklin, '05.) (A) Egg shortly after spawning but before sperm entrance. 

 The spindle fibers of the first maturation division are forming, and the nucleoplasm is 

 located toward the animal pole. (B) Egg showing the spindle for first maturation divi- 

 sion. Observe the sperm nucleus just inside the ooplasmic membrane near the midvegetal 

 pole of the egg. The nucleoplasm (karyoplasm) of the female nucleus has spread into a 

 thin cap at the animal pole. (C) Metaphase of first division spindle (1, D.S.) nearly 

 parallel to the surface of the egg; no centrosomes are present. (D) Higher powered 

 representation of sperm a little later than that shown in (B). The aster for the first 

 cleavage spindle is forming in the middle piece of the sperm. (E) Slightly more ad- 

 vanced stage than that shown in (B). Collection of yellow-pigmented, peripheral proto- 

 plasm (PL.) is shown at bottom of the egg. (F) Anaphase of second polar spindle. 

 Sperm aster enlarging. (See (G) and (H).) (G) Separation of first polar body. (H) 

 Metaphase of second polar spindle, paratangential in position. (I) First polar body 

 (1 P.B.) formed; chromatin of second spindle (CHR.). Sperm has revolved 180°; sperm 

 aster enlarging. (J ) Telophase of second polar spindle. Sperm aster enlarges, and sperm 

 nucleus assumes vesicular condition. (K) Separation of second polar body. (L) Two 



{Continued on facing page.) 



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