448 Gregory . — Spore- Formation in Leptosporangiate Ferns . 
As Farmer and Moore have pointed out 1 } the increasing difficulty of 
recognizing the original longitudinal fission in the limbs of the chromo- 
somes during these successive stages 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 
very much shortened and thickened chromosomes. A similar conception 
led to the interpretation of the c ring 5 type 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 recognized in each limb of the chromo- 
some (Figs. 17, 18, 20, 21). In the same way favourable preparations 
reveal the double nature of the 4 ring ’ chromosomes, which, as has been 
pointed out, differ from the other forms only in the degree to which the 
approximation of the limbs towards one another has been carried 2 . 
The chromosomes may acquire a striking resemblance to the tetrads 
which are characteristic of the heterotype division of certain animals 3 . 
The resemblance becomes particularly marked at about the time when the 
nuclear wall breaks down (Figs. 22 and 23), and is due only to a more or 
less temporary splaying of the ends of the chromosomes (compare Figs. 
16-23 with those of Calkins, Lc., PI. 295, Fig. 6, I, j, K). Calkins’s inter- 
pretation of the processes resulting in the tetrad-like appearance as 
indicating the transverse fission of the chromosomes is therefore incorrect. 
The spindle-fibres are attached to the limbs of the chromosomes near 
the distal ends of the latter (Figs. 22, 23, 24); as the 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 this transverse fission, which separates the two 
limbs of each loop, takes place is not easily determined and appears to be 
variable. In many cases it appears to have been completed before 
metaphase is reached, so that the chromosomes as they move toward 
the equatorial plate each consist of two separate parallel rods which repre- 
sent the limbs of the original U (Fig. 23) 4 . In others the separation only 
becomes visible later and appears to be synchronous with the commence- 
ment of the contraction of the spindle-fibres, and consequent divergence 
of the limbs of the chromosomes (Figs. 24, 24 a) 5 . 
During the latter part of the period just described-— as the chromo- 
1 Farmer and Moore, 1. c. 
2 Compare Figs. 17-21 with those of Calkins, 1. c., PL 295, Figs. 4 and 5. 
3 Wilson, 1. c., p. 246 et seq. 4 Cf. Calkins, 1. c., PL 295, Fig. 6, A, B. 
6 Cf. Calkins, 1. c., PL 295, Fig. 6, 1, j, K. 
