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



the uterine spindle. He describes it as appearing in several 

 species of polyclads; and while he does not accept Selenka's 

 theory as to its function, nevertheless he regards it as a part of 

 the normal process of the nucleus. 



In 1894 Wheeler observed the uterine spindle in the eggs of 

 Planocera inquilina, a polyclad inhabiting the branchial chamber 

 of Sycotypus canaliculatus . Wheeler was concerned with a 

 description of this new polyclad, and did not attempt to work out 

 the details of the karyokinetic process. Since Planocera is the 

 form with which this paper is concerned, we may quote Wheeler's 

 entire but brief statement on the aborting spindle. His state- 

 ment is as follows: "As soon as the mature ova pass into the 

 uteri a curious phenomenon, first seen by Selenka in the uterine 

 eggs of Thysanozoon Diesingii, may be observed. The wall of 

 the germinal vesicle fades away and a spindle is formed with 

 distinct polar suns containing centrosomes. The small chromo- 

 somes, nine or ten in number, form an equatorial plate and 

 appear to undergo fission, but of this I am not certain. Then the 

 polar asters grow faint and vanish and the nucleus returns to 

 the resting stage during or just before oviposition. Before the 

 nucleus has returned to the resting stage the spermatozoon 

 enters the egg. I have several times seen the deeply staining 

 and somewhat twisted head of the spermatozoon lying in the 

 cytoplasm near the arrested spindle. Further than this I have 

 not traced the phenomena of impregnation, as my attention was 

 first attracted to them while studying hardened material when 

 I was far from the sea-shore. Why a spindle should be formed 

 in the mature ovum and no division result, but only a return of 

 the nucleus to its resting stage, is not easily understood. The 

 spindle lies in the center of the egg and has nothing to do with 

 the formation of the polar bodies; for these do not appear till 

 some time after the eggs are laid, as I have several times had 

 occasion to observe." Wheeler concludes his account of the 

 spindle by objecting to Selenka's view concerning its supposed 

 function. 



Gardiner, '95 and '98, studied the uterine spindle in the 

 accelan Polychcems caudatus; but in this form it is clearly the 

 first cleavage spindle, as the polar bodies are thrown off before 



