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



in which is seen what he takes to be a "germinal vesicle" pro- 

 duced by the retrograde development of the uterine spindle. 

 Surface states that if the spindle is an abnormal display, as 

 claimed by Gardiner, it at any rate does not interfere with the 

 normal process of development. 



The last account of the uterine spindle is that on the rhab- 

 doccele Graffilla Gemellipara, which was studied by one of the 

 present writers (Patterson, '12). In this animal a large con- 

 spicuous spindle was occasionally met with in eggs that had not 

 yet undergone maturation. The spindle in Graffilla differs from 

 that so far observed in any other worm, for in practically every 

 case it exhibited some peculiar condition, such as the abnormal 

 position of the chromosomes on the spindle fibers, or even their 

 complete absence from the spindle. It was pointed out that on 

 account of the viviparous mode of reproduction, Graffilla was not 

 a favorable form in which to study the history of the aborting 

 spindle. It is impossible to secure a complete series which 

 would show conclusively the exact progress of its development. 



To sum up: The term uterine spindle has been applied to a 

 variety of karyokinetic phenomena which occur during the 

 first steps of development. Some of these cases are undoubtedly 

 due to abnormal development; others are not, especially those of 

 the polyclads. Here the general verdict seems to be that the 

 uterine spindle appears before maturation, that it does not go 

 beyond the equatorial-plate stage, and that it subsequently 

 retrogrades to produce a sort of resting nucleus, which in turn 

 develops the first maturation spindle. 



It is evident from the above brief review of the literature that 

 a great deal of obscurity exists regarding the uterine spindle. 

 In view of the fact that no one has given a consistant account of 

 its development, and in view of the further fact that no rational 

 function has been assigned to it, we have considered it worth 

 while to make a detailed study of this spindle. To do this we 

 have selected Planocera, not only because of the ease with which 

 this animal can be secured, but also for the reason that its egg 

 is supposed to contain the most typical example of this apparent 

 anomaly of cytology. 



Material and Methods. Two or three is the most common 



