UTERINE SPINDLE OF PLANOCERA INQUILINA. 273 



it makes its appearance. However, according to the description 

 of Gardiner the behavior of this spindle is similar to that of the 

 uterine spindle of the polyclad egg. Thus he states that if the 

 animal be kept too long under adverse conditions, the polar suns 

 of the spindle grow dimmer, draw closer together, and the 

 nucleus appears to return completely to a resting stage. The 

 egg remains in this condition until after it is laid, when the 

 spindle again appears, this time to initiate the process of cleavage. 



Through certain experiments, Gardiner, '98, demonstrated 

 that the retrograde growth of the amphiaster of the uterine egg 

 was due to placing the animals under adverse conditions, which 

 in turn caused a delay in the laying of the egg, and under such 

 circumstances development begins, as indicated by the appear- 

 ance of this spindle. Furthermore, he clearly showed that the 

 uterine spindle of Poly cheer us follows maturation, and is there- 

 fore, as stated above, the first cleavage spindle. Through the 

 results of these experiments, Gardiner is led to suggest that the 

 so-called uterine spindle of the polyclads is probably the first 

 segmentation spindle. His exact position on this point may be 

 gleaned from the following quotation: "I would suggest, there- 

 fore, that individuals of the polyclads, in which such structures 

 are found, have before death been placed under some abnormal 

 conditions; that the ovum has been fertilized and the polar bodies 

 formed ; that the first segmentation spindle has been formed ; and 

 that the environment was such that oviposition could not take 

 place; consequently, that a retrograde development of this 

 spindle has taken place exactly as in Polychcerus" 



That the ground for this suggestion is not well taken, we shall 

 later try to demonstrate, at least in the case of Planocera. The 

 suggestion of Gardiner, however, should make us cautious about 

 regarding as aborting spindles those that are visible in the uterine 

 eggs of several worms figured by von Graff ('82 and '08) and 

 which a number of investigators have cited as examples of the 

 disappearing spindle. 



In 1907, Surface, who studied the early development of 

 Planocera, also called attention to the uterine spindle in this 

 animal. He did not attempt to work out the history of the 

 spindle, but gives merely an outline figure of a freshly laid egg, 



