Vol. XXIII. October, 1912. No. 5 



BIOLOGICAL BULLETIN 



THE UTERINE SPINDLE OF THE POLYCLAD 

 PLANOCERA INQUILINA. 



J. T. PATTERSON AND H. L. WIEMAN. 



I. INTRODUCTION. 



In the uterine eggs of several species of turbellarians there 

 appears a large conspicuous spindle to which the name "uterine 

 spindle" is applied. It is also known under the terms "dis- 

 appearing spindle" and "aborting spindle." The appearance 

 of this spindle in the uterine eggs of these worms would not in 

 itself be so striking were it not for the remarkable statements 

 that have been made concerning its subsequent history. It is 

 the general opinion of those who have observed the spindle that 

 upon reaching the metaphase it breaks down to form a resting 

 nucleus, which in turn gives rise to the first maturation spindle. 



The uterine spindle was first described in 1881 by Selenka, in 

 connection with his work on Thysanozoon Diesingii. He states 

 that it first makes its appearance in the eggs after they have 

 become full grown and have passed into the uterus, and is pre- 

 ceded by changes that are typical of the first maturation mitosis. 

 Thus the chromatin of the germinal vesicle forms a spireme, the 

 achromatic spindle with centrosomes appears, and the chromo- 

 somes pass into the equatorial-plate position. At this point the 

 division process is arrested, and the "polar-suns" draw closer 

 together, become indistinct, and the chromosomes fuse. The 

 whole nucleus finally returns to a resting condition. Later, the 

 egg gives off two polar bodies, is fertilized and proceeds to develop 

 in the normal manner. Selenka concludes that this incomplete 

 karyokinesis occurs in order to effect a massing of the yolk 

 granules about the astral centers. 



Lang ('84) seems to have been the next observer to have noted 



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