122 H. H. NEWMAN. 



and others as high as 19. The larger number may be, and 

 probably is, due to the precocious separation of a few of the 

 double elements in the early anaphase. The lower count may be 

 due to some elements being fused with or hidden under others. 

 Personally I am convinced that the normal reduced number of 

 chromosomes in the female of this species is 16, since this number 

 has occurred as often as all others combined. 



Fig. 34 shows a rather unusual spindle in which all of the 

 chromosomes are in the form of well-defined tetrads, in some 

 cases just separated into diads. A number of other spindles 

 have been found where some of the elements were typical tetrads 

 of this sort, but others were merely double in appearance. In 

 the anaphases, a good view of which is given in Fig. 35, the 

 chromosomes are clearly diads of dumbell shape. In late ana- 

 phases, as in Fig. 36, these diads assume the flattened shape char- 

 acteristic of the tetrads of the prophase and form ring- or disc- 

 shaped masses at either end of the barrel-shaped spindle. The 

 band-shaped Zwischenkorper forms a characteristic feature of 

 this phase of the division. 



During the prophases and anaphases of polar body formation 

 the spindle lies in a parallel or tangential position with reference 

 to the cell membrane and it is only in the last steps of the ana- 

 phase that any sign of polar extrusion appears. The polar body 

 begins to form after the manner shown in Fig. 36, by the appear- 

 ance of a slight furrow, like the beginning of a cleavage furrow. 

 As this furrow deepens the spindle assumes a position more nearly 

 perpendicular to the surface of the ovocyte and the polar body 

 is constricted off as in Fig. 37. 



IX. THE FIRST POLAR BODY AND THE SECOND POLAR SPINDLE. 



Only eight ovocytes with one polar body have been found in 

 the present study as compared with literally hundreds with first 

 polar spindles. This fact would seem to suggest that the 

 majority of the ovocytes come to an equilibrium in the metaphase 

 of the first maturation division, and require some special stimulus 

 to cause them to complete this division. It is very probable that 

 the stimulus needed is that brought about by mating. If we may 

 infer from analogy with other mammalian studies of ovulation 



