28 THE MATURATION OF THE EGG OF THE MOUSE. 



Fully formed chromosomes are shown in plate A, fig. G (e to /) and in 

 figs. 6 to 14 (plates 1 to 3). In fig. G a typical chromosome is shown at / 

 in face view {i.e., looking toward the axis of the spindle along that 

 radius of its equator which passes through the middle of the chromo- 

 some), and at e in side view {i.e., looking in the direction of the tangent 

 to the equator of the spindle which cuts the chromosome at its middle 

 point). To the pointed ends of the chromosome are attached the spindle 

 fibers; the side of e (fig. G) which is directed to the right is that which 

 is turned away from the axis of the spindle. The chromosome is com- 

 posed of four deeply stained parts, which are more or less completely 

 separated from one another by two deep constrictions, one longitudinal, 

 the other, less complete, transverse. In a sense the separation is incom- 

 plete in both directions, because the four deeply stained parts are con- 

 nected to one another by a less deeply stained substance, in which they 

 are, as it were, embedded. This substance may possibly be in part non- 

 chromatic, but probably it contains a certain amount of chromatin. This 

 diminution in the proportion of chromatin is also evident at the pointed 

 ends of the chromosomes, where, as already stated, the spindle fibers are 

 attached (see e and /). The chromosome illustrated by the two views 

 g and h differs from that seen in e and f chiefly in being more elongated, 

 the four median, deeply stained regions of h being the upturned adjacent 

 ends of the four parts resulting from the elongation of the correspond- 

 ing thicker four parts shown in /. In both these cases the transverse 

 division is less conspicuous than the longitudinal. In j both divisions 

 are obscured by the temporary fusion or adhesion of the four parts. 

 The cross-division is, however, represented by a constriction. To one 

 or other of these three conditions can be referred all the other forms of 

 the fully developed chromosomes, the differences being due merely to 

 various degrees of fusion or separation of the parts. All of these chro- 

 mosomes ultimately he with their long axes approximately parallel to 

 that of the spindle. 



We return now to the fundaments of the chromosomes and their 

 development into the forms last described. It is to be noted that in the 

 early stages (figs. 2 and 3) some fundaments show only a single (longi- 

 tudinal) division. The transverse division, seen clearly in the left-hand 

 chromosome of fig. 4a, arises a little later, as may be inferred from the 

 condition shown in the lower right chromosome of fig. 4a and in the 

 lower (pale) chromosome of fig. 5 ; this division may perhaps arise much 

 later. The 4-part condition appears to be a typical one. When it per- 

 sists as late as the time of the formation of the spindle, the chromo- 

 some generally lies with its long axis parallel to that of the spindle (fig. 

 5). Were there no forms intermediate between this and the one shown 

 in f (fig. G), the four parts of the one might be referred in all cases di- 

 rectly to the corresponding parts of the other. But the forms b and c 

 {d answering for the face view of both b and c) are apparently intermedi- 



