1904.] 



The Reduction Division in Ferns. 



87 



As the polarity of the spireme becomes more pronounced the limbs 

 of each loop approach each other, and segmentation into the chromo- 

 somes takes place. Each chromosome has its origin in one of the 

 loops of the spireme and thus forms a double U-shaped body, the 

 limbs of the U being twisted upon one another to varying degrees in 

 the different chromosomes of the same nucleus. The approximation 

 towards one another of the distal ends of the limbs of each U, often 

 resulting in the appearance of a " ring " chromosome, is a common 

 feature of the heterotype division in Ferns. 



As prophase leads up to metaphase the original longitudinal fission 

 becomes more obscure, and as the chromosomes begin to group them- 

 selves in the equatorial plate, each appears to consist of two parallel 

 rods, which represent the approximated limbs of each loop of the 

 spireme, and are joined at one end. 



The increasing difficulty during these successive stages of recognising 

 the original longitudinal fission in the limbs of the chromosomes has 

 led to an incorrect interpretation of their structure. The two limbs, 

 of which each chromosome consists, were interpreted as being the result 

 of the original longitudinal fission in the now shortened and thickened 

 chromosomes. A similar conception led to the interpretation of the 

 " ring " form of chromosome as being due to the divergence of the 

 halves into which each chromosome was separated by that fission. 

 The examination of numerous preparations of the stages intermediate 

 between that of the looped spireme and that of early metaphase 

 reveals the incorrectness of this interpretation, inasmuch as the 

 original longitudinal fission can be clearly recognised in each limb of the 

 chromosome. In the same way favourable preparations of the " ring " 

 type of chromosome reveal the double nature of the ring, while in 

 many cases the distal ends of the limbs either do not quite meet, or 

 on the other hand may overlap, thus providing forms transitional to 

 the U and X types of chromosome. 



The spindle fibres are attached to the limbs of the chromosomes 

 near the distal ends of the latter; as, therefore, the two daughter- 

 chromosomes are drawn apart, the familiar ^-shaped figures are 



obtained, and the final separation takes place at a point corresponding 

 with the apex of the original loop. 



The exact time when the transverse fission, which separates the 

 two limbs of each loop, takes place is not easily determined and 

 appears to be variable. In some cases it appears to have been com- 

 pleted before metaphase is reached, so that the chromosomes as they 

 move to the equatorial plate consist of two separate rods. In others, 

 on the contrary, the separation is synchronous with the commence- 

 ment of the contraction of the spindle fibres, and consequent divergence 

 of the limbs of the chromosomes. 



