UTERINE SPINDLE OF PLANOCERA INQUILINA. 279 



are easily explained on the basis of the assumption that there is 

 considerable variation in the time of oviposition on the part of 

 the different individuals; and this in turn is undoubtedly in- 

 fluenced by the conditions under which the animal is kept just 

 prior to the laying of the eggs. 



We have been somewhat at a loss to account for the obser- 

 vations of Wheeler and Surface, but believe that they may be 

 explained in any one of several different ways. If a lot of 

 freshly laid eggs, in which the contraction of the spindle has 

 progressed to an advanced stage, be examined under the low 

 or medium powers of the microscope, many of the eggs will 

 appear to possess germinal vesicles. However, it can be shown 

 conclusively that under such conditions the small contracted 

 spindle is practically invisible, and that what one really observes 

 in these living eggs is the relatively clear area of protoplasm in 

 which the small spindle lies. This can be shown beautifully by 

 staining the fresh eggs with neutral red, and examining them 

 under the 4 mm. objective. Under such conditions, the con- 

 tracted spindle stands out with great brilliancy, and one can 

 easily follow the course of its migration to the surface of the 

 egg and observe the formation of the first polar body. 



That eggs possessing germinal vesicles may be laid we do not 

 deny, for occasionally they are; but we can affirm that such 

 vesicles are never formed in an egg after it has produced the 

 uterine spindle. They are merely the non-transformed germinal 

 vesicles of ovarian eggs, and their presence at this stage is to be 

 explained by the fact that eggs possessing them either have 

 recently been penetrated by the spermatozoon, or have not been 

 inseminated at all. There is no room for doubt on this point. 

 These germinal vesicles are in every particular similar to those 

 of the ovarian egg so much so that we have deemed it un- 

 necessary to draw one for illustration, but refer the reader to 

 Fig. i. Furthermore, we have found at least two freshly laid 

 eggs which contained germinal vesicles undergoing transformation 

 to form the first maturation spindle. Each of these eggs showed 

 a condition that indicated recent insemination, for the sper- 

 matozoon was lying close to the egg membrane and had not yet 

 undergone the transformation necessary to produce the vesicular 



