UTERINE SPINDLE OF PLANOCERA INQUILINA. 2/7 



in outline suggesting that certain substances are passing out of 

 the nucleus into the cytoplasm. 



Fig. 4 shows the chromosomes scattered irregularly on the 

 spindle, and it will be noted that the chromosomes are of a 

 bivalent type. The centrosome is seen at one pole as a deeply 

 staining sphere, in the astral area of which the spermatozoon 

 lies. The section does not pass through the centrosome of the 

 other pole. The chromosomes quickly move to the equatorial 

 region and the spindle remains in this condition until the egg is 

 laid. Fig. 5 is a polar view of such a spindle and shows the 

 number of chromosomes to be ten, a number confirmed by many 

 counts. Fig. 6 represents the characteristic appearance of a 

 section of a uterine egg showing the entire spindle with the 

 chromosomes in the equatorial plate. The spermatozoon is 

 present in the next section of this egg. 



Such in brief is the history of the formation of the uterine 

 spindle. It is an enormous structure occupying the entire central 

 region of the egg; its astral radiations stretching out from either 

 pole almost to the egg membrane. The centrosomes appear as 

 deeply staining spheres, in iron-haematoxylin preparations, but 

 in safranin-Lichtgriin each sphere is resolved into a number of 

 hollow vesicles staining with the acid dye. 



At this time the spermatozoon may be seen in almost any part 

 of the egg, between the spindle and the periphery. It may lie 

 in the astral area or very close alongside of the spindle and near 

 the chromosomes. The spermatozoon shows no change in 

 structure from the sickle-shaped form in which it first appears in 

 the impregnated ovum. 



This is the type of spindle formed in the vast majority of 

 uterine eggs, and in animals secured under the best conditions, 

 only this type is found. However, in some adult worms, in 

 addition to this so-called normal spindle, there occur other 

 spindles which have the same general structural features, but 

 show anomalies of various sorts. Thus the axis of the spindle 

 may be bent (Fig. 7) or even broken at the equator and the 

 chromosomes may be scattered irregularly on or near the spindles. 

 Tri- and tetra-polar spindles of a variety of forms are also found, 

 two of which are shown in Figs. 8 and 9. None of these spindles 



