OBSERVATIONS ON THE MATURATION PROCESSES. 37 



ence, and relative position (figs. 23 to 27, plates 4, 5; fig. /). Fig. / 

 (m to r) illustrates several of these variations. In the simplest form of 

 chromosomes, shown at m, each element is a straight rod, either of uni- 

 form size (see also fig. 25), or slightly constricted in the middle. The 

 constriction is likewise evident in chromosomes seen when looking 

 nearly in the direction of the axis of the spindle (figs. 20, 21). Viewed 

 under these conditions usually one element of the pair is partially cov- 

 ered by the other. Even after the separation of the daughter chromo- 

 somes from each other, this constriction or dumb-bell condition of the 

 daughter chromosome is evident, whether seen endwise (fig. 28) or in 

 side view. In p (fig. /) the constriction is carried still further, some- 

 times to such an extent that the mother chromosome appears to be com- 

 posed of four nearly independent parts (x, fig. 24a). Sometimes the 

 daughter chromosomes are curved rods (fig. /, r; plate 5, figs. 246, 26), 

 or are of an irregular crescent shape (0) . Fusion or adhesion of the two 

 elements at one or more points gives rise to figures like n (see also figs. 

 20, 23a, x). When the elements are more elongated and curved, rings 

 (q, also figs. 24a, 25, 26) are formed by the fusion of the corresponding 

 ends of the two daughter chromosomes. Occasionally, when the fusion 

 of the ends (as in n) is well advanced and the constriction in the middle 

 of each is complete, the original 

 separation between the two ele- ^_^ ^^ a ir, „ |B» JBm § 



SS wC o\ 



ments is obscured and the mother 



chromosome then appears to be 



composed of two parts, the long m n ° V V r 



axes of which are perpendicular to Fig. I. 



the plane of the equator. Forms 



like those shown in figs. 25 and 2 7, which occur in eggs that have remained 



long in the oviduct, are explained by the fact that with age the elements 



tend to elongate. In any of these forms of chromosome the parts may 



be parallel to each other, or, according to the point at which the spindle 



fibers are attached, separated at one end (n, p, r) or at the middle (p, q). 



The chromosomes are never arranged at the surface of the spindle, 

 but from the beginning are uniformly distributed in the plane of its 

 equator (figs. 20 to 24), and are so oriented that that plane passes be- 

 tween the two elements of each mother chromosome. This arrangement 

 of the daughter chromosomes in one plane is preserved even after meta- 

 kinesis (figs. 28a and 286). 



The number of chromosomes is 20; but the proportion of cases in 

 which the number can be determined with accuracy is smaller than in 

 the case of the first spindle, because in the second spindle the chromo- 

 somes are more crowded and their forms are less regular than in the 

 first spindle. When, in cutting, the chromosomes fall in two sections 

 the difficulty of counting is usually increased. However, knowing the 

 structure of the chromosomes, it has been possible in many cases to be 



