KARYOKINESIS. 



♦ ♦V 



ti- 

 ll* 



reduced number of chromosomes ; I have been unable to count with accuracy the 

 number present after fertilization, but it is evidently about sixty, 



In the early anaphase of the first maturation, the daughter chromosomes are 

 either dumb-bell shaped, or cubical or spherical masses, frequently with a slender 

 process of chromatin running from each daughter chromosome toward the other, 

 figs. 13-15 and text fig. I. A little later each chromosome becomes cubical or 

 quatrefoil in shape, and this form persists, and is universal until the metaphase of 

 the second maturation division, when all become 4-parted, figs. 16-31 and text fig. 

 II, and then cross shaped exactly as in the first maturation. In this case, however, 

 the arms of the cross are not split longitudinally, but the separation takes place 

 between the arms, so that in the metakinesis two of the arms or spherules, in the 

 form of a dumb-bell, go to one pole and two to the other, text fig. II, 5 and 6. In 

 the anaphase of the second maturation, these dumb-bell or rod-shaped chromosomes 

 again become cubical or spherical, as in the anaphase of the first maturation (Plate 

 II, figs. 32, 33 and text fig. II, 8). 



The cubical chromosomes at the close of the first maturation are about half 



as large as the fully-formed chromosomes in 

 the metaphase of that division, while those 

 in the anaphase of the second maturation 

 are about one-half the size of those in the 

 prophase of this division, i. <?., the volume 

 of each chromosome at the end of the sec- 

 ond maturation division is about one-quarter 

 that of the fully -formed chromosome in the 

 metaphase of the first maturation. There 

 has been therefore no growth of the chromosomes after the metaphase of the first 

 maturation. The number of chromosomes in the second maturation division is 

 the same as in the first, viz. thirty, and the same number is left in the egg after 

 the second polar body has been formed. 



It is especially noteworthy that in the prophase and anaphase of both matura- 

 tion divisions the chromosomes are cubical or quatrefoil in shape. In the metaphase 

 of the second maturation, figs. 27-31 and text fig. II, the chromosomes look like 

 typical "tetrads" and they would undoubtedly be called such if they occurred in 

 the first maturation. 



Similar 4-parted chromosomes in the second maturation are figured by v. Klinc- 

 kowstrom ('96) in Prostheceraeus, by Van der Stricht ('98) in Thysanozoon, and by 

 Byrnes ('99) in Limax. 



In Crepidula it is impossible to say whether the plane of division of the chro- 

 mosomes is the same or is different in the two maturation divisions. At the begin- 

 ning and at the end of both divisions the chromosomes are cubical or quatrefoil in 

 shape, and one might as well speak of the "longitudinal" or "transverse" division 

 of a cube or sphere as of these chromosomes. It is impossible, therefore, to deter- 

 mine whether or not reduction in the sense of Weismann takes place in this case. 



Fig. II.— Ch 

 Divisi 

 this fi( 



of the Second Maturation 

 l of Crepidula. (In the reproduction 

 ire has been reduced more than fig. I.) 



