12 KARYOKINESIS. 



given mitosis cannot be reduced to a single type; that their differences in shape 

 indicate differences in material substance, and that different chromosomes may 

 therefore represent different heritable qualities. On the other hand these differ- 

 ences in the shapes of chromosomes are generally limited to the first maturation 

 division ; they are rarely found in the second maturation and onlj^ to a limited 

 extent in the cleavage. Furthermore, there are many evidences that the shapes of 

 chromosomes are conditioned by their linin sheaths, and that the chromatic substance 

 which fills the sheath is of a semi-tluid of viscid character. Thus in the metaki- 

 nesis of the first maturation, it is always found that the chromosomes have enlarged 

 ends toward the poles of the spindle, and that they are drawn out into thin connect- 

 ing threads in the equator. In this region the chromosomes are frequently monili- 

 form in shape (text fig. I, C, 8 and 9 and Plate I, fig. 13), and cross sections through 

 the equatorial region of these elongated chromosomes shows many of the latter sur- 

 rounded by a clear zone, which is bounded by a dark line (Plate I, fig. 12a). This 

 clear zone is entirely lacking in sections through the enlarged ends of the chromo- 

 somes. In fig. 12a one chromosome lies entirely outside of the spindle substance, 

 and yet it is surrounded by this clear zone; this zone and its outer dark boundary 

 is not therefore a mere exjDression of the absence of spindle substance around the 

 chromosome, but is a structural peculiarity of the chromosome itself, and probably 

 represents a linin sheath, which is separated from the contained chromatin in the 

 equator, but is entirelj^ filled by the chromatin at the poles. After the complete 

 division of this thread of chromatin and its withdrawal into the enlarged ends of 

 the daughter chromosomes, the linin sheath may still be seen for a long time con- 

 necting the latter together, and constituting a connective fibre. 



The chromosomes grow continually during the early stages of the first matura- 

 tion division and reach their greatest size in the metaphase when each is from two 

 to twenty times the size of the largest granule present in the germinal vesicle (cf. 

 figs. 3 and 12). After this stage they do not appear to increase in volume. The 

 great differences in the size of chromosomes in the same spindle is almost as striking 

 as their differences in shape ; for example, the volume of the largest 3-part chromo- 

 some in Plate I, fig. 12 and text fig. I, B, 7 and 8, is about fifteen times that of the 

 smallest ; I am unable to say, however, whether this difference in the size of chro- 

 mosomes is the same in all eggs or not. I do not remember that any one has 

 recorded such enormous differences in the size of chromosomes as are here described. 

 Montgomei-y ('98) says that the chromosomes of one of the spermatocytic divisions 

 in Pentalonia vary greatly in volume, the largest sometimes having six times or 

 more the volume of the smallest. In no other mitosis in Crepidula is there such 

 variety in the size of chromosomes, and nowhere else are there such differences in 

 shape. 



The number of chromosomes in the first maturation division is thirty, as I have 

 determined by a careful study of the entire mitotic figure, as well as by cross sections 

 through the equatorial plate. Such a cross section is represented in Plate I, fig. 12a, 

 and the whole number of chromosomes is there shown. This is undoubtedly the 



