30 THE MATURATION OF THE EGG OF THE MOUSE. 



plate "are therefore unsatisfactory for counting chromosomes. It is 

 an interesting fact that in the spindles drawn in figs. 10 and n (plate 

 2) the chromosomes lie nearer that end of the spindle which is more 

 pointed and about which the evidences of cytoplasmic radiations are 

 more pronounced. 



The chromosomes are oriented with their long axes parallel to the 

 long axis of the spindle. The few exceptions may in some instances be 

 natural, but in others they certainly are due to displacement by the 

 knife in cutting (figs. 12, 136, x and x'). 



The separation to form the daughter chromosomes always takes 

 place at the middle of the chromosome and at right angles to its long axis 

 (plate A, fig. G, / to /). While, in general, all the daughter chromosomes 

 migrate toward the spindle poles at the same time (fig. 15), it sometimes 

 happens that one or more of the chromosomes divides and the halves 

 move apart at an early stage before their sister chromosomes show any 

 signs of migration (two pairs in fig. 9). In the latter case the precocious 

 daughter chromosomes show no longitudinal division, while in the former 

 they are clearly split lengthwise (plate A, fig. G, i, I; plate 3, fig. 15). 

 Fig. 1 5 shows a spindle which is nearly parallel to the surface of the egg ; 

 in this case each daughter chromosome consists of halves, each of which 

 is elongated and somewhat tapering, the narrower end being directed 

 toward the pole of the spindle; the halves are parallel to each other or 

 slightly converging toward the ends which point to the pole. In another 

 spindle, of like age but occupying a radial position in the egg, the halves 

 of each daughter chromosome are in contact at their polar ends, but 

 widely separated at the equatorial end, thus together forming a distinct 

 V. In fig. 17 the daughter chromosomes are more compact, and fewer 

 show the longitudinal division. Some of them are much more elongated 

 than others. The spindle in plate 3, fig. 16, being cut obliquely, shows the 

 daughter chromosomes more clearly. The two limbs of each daughter 

 chromosome are easily distinguishable, each being somewhat dumb-bell 

 shaped. The two lie side by side, and in some cases by bending assume 

 the form of flattened rings (fig. 166). Later the chromosomes at each 

 end of the spindle fuse into a compact, deeply staining, disk-shaped, or 

 sometimes cup-like, mass (plate 4, fig. 18). 



In spite of the differences of opinion which have been expressed 

 concerning the number of chromosomes, we think there can be no doubt 

 that typically in the animals we have studied it is 20. A knowledge of 

 the structure of the chromosomes makes it possible in many cases to 

 be absolutely sure that this is the number. Table 2 gives the results of 

 our observations on this subject. The accuracy of the counting depends 

 on the stage of the spindle and the position which it occupies with respect 

 to the plane of cutting. When the chromosomes are scattered along the 

 spindle (figs. 6, 7, and ja), they obscure one another least and frequently 

 can be counted with perfect accuracy. Upon the formation of the 



