$8 THE MATURATION OF THE EGG OF THE MOUSE. 



quite certain that the number is 20 (see table 2, p. 14). In figs. 23a and 

 236 there are only 19 chromosomes, one probably having been lost in 

 cutting. Figs. 24a and 24b exhibit together 20, one having been so cut 

 that a half of it lies in each section. The two sections (24a and 24b) 

 contain, respectively, 8.5 and 11.5 chromosomes. Polar views of the 

 "equatorial plate" are usually the most satisfactory ones for counting. 

 In fig. 20, a polar view, there are clearly 20 chromosomes; one of these 

 (x), seen in face view, corresponds to fig. /, n. In figs. 28a and 286 (an 

 anaphase) the number can not be determined with perfect accuracy, 

 because the long axes of the daughter chromosomes are perpendicular 

 to the plane of the section. Two of the larger chromosomes (x and x') 

 may well be double; if so, the number in this case also is 20. 



In the division of the chromosomes, the two elements of each 

 mother chromosome separate and then migrate to the opposite poles 

 of the spindle. Figs. 28a and 286 (plate 5) are polar views of the two 

 daughter plates at a stage of migration corresponding to that of fig. 16, 

 and are drawn from a non-seminated egg. In fig. 29, which represents 

 a slightly later stage than fig. 28, the individual chromosomes are no 

 longer distinguishable. They seem quickly to lose their identity and 

 merge into a single disk-shaped mass (fig. 30), as in the case of the first 

 spindle. 



3. ACHROMATIN PARTS OF SECOND MATURATION SPINDLE. 



The interzonal filaments left in the egg after the first polar cell is 

 cut off persist for a while along with the chromatin mass. About the 

 time when the chromatin breaks up into fragments, they lose their con- 

 nection with the cell plate (plate 4, fig. igb). It is probable, but not cer- 

 tain, that they contribute to the formation of the matrix in which the 

 chromatin fragments are embedded, and also to the formation of the 

 fibers of the completed second maturation spindle. 



The second spindle begins as a somewhat pear-shaped, apparently 

 homogeneous body at the time when the chromatin mass divides into 

 fragments. When completed it is more or less elliptical (fig. 22), like 

 the first spindle, but it varies more in form than does the first spindle, 

 being occasionally more slender and having more sharply pointed ends. 

 However, as observed from the surface of the egg, it often appears very 

 broad (figs. 23, 24), owing to its being flattened in the direction of the 

 radius of the egg (fig. 20). Such spindles when seen edgewise appear 

 very narrow (fig. 22) ; they always lie nearer the surface of the egg than 

 those which are circular in cross-section. 



The fibers of recently formed spindles resemble quite closely those 

 of the later stages of the first spindle in being smooth, of uniform 

 diameter — except at their polar ends, where they are thickened — and 

 curved inward toward the poles (figs. 22 and 24). The thickenings at 

 the polar ends are not to be seen in fig. 23 , because the spindle was stained 

 in Bohmer's haematoxylin and Congo red, which are not favorable for 



