Gregory . — -Spore- Formation in Leptosporangiate Ferns . 447 
3. Observations. 
The chromosomes of the somatic mitoses are so crowded that it Is not 
easy to determine their exact number. In the species of the Polypodiaceae 
which were examined it is possible to count about 60 chromosomes ; no 
evidence was obtained of the occurrence of a number approaching that 
given by Calkins for Pteris tremula and Adiantum cuneaium (130- 130). 
The reduced number of chromosomes is 3 2 ; there is therefore a strong 
presumption that 64 chromosomes are present in the somatic mitoses, as 
stated by Stevens. In Alsophila the number of chromosomes is larger, the 
reduced number apparently being about 60. 
After the vegetative divisions of the archesporium are complete the 
spore-mother-cells undergo a period of rest and growth. The first 
indication of their approaching division is the transformation of the reticulum 
of the nucleus into a much-coiled spireme thread, which at this stage Is 
evenly distributed throughout the nucleus (PI. XXXI, Fig. 1). The 
spireme now undergoes a longitudinal fission (Fig. 2) ; the two halves of 
the divided thread generally lie close to one another, but may diverge 
to a certain extent in some places (Fig. 7). The fission of the thread is 
quickly followed by its contraction towards one side of the nucleus, which 
is accompanied by a pulling-out of the thread into a series of loops 1 (Figs. 
3 , 4 and 5). 
As the polarity of the spireme becomes more pronounced the limbs of 
each loop approach one another (Figs. 6, 7, 11-14) and in many cases 
become closely applied to, or even twisted upon one another. In the 
earlier stages of this process, the double nature of the chromatin thread 
is quite clear (Figs. 6-14), and is revealed even in the least satisfactory 
preparations if by chance the thread has been cut across (Fig. 10). In the 
later stages this structure becomes more obscure, but in some, at least, 
of the loops of any nucleus indications of the double nature of each limb 
can be found (Figs. 12, 13, 14). 
The segmentation of the longitudinally-divided thread into chromo- 
somes takes place in such a way that each chromosome has its origin in one 
of these loops 2 , and thus forms a U-shaped body, the limbs of the U being 
twisted upon one another to varying degrees In the different chromosomes 
of the same nucleus (Fig. 15). The approximation towards one another of 
the distal ends of the limbs of each U, often resulting in the appearance 
of the ‘ring 5 type of chromosome, is a common feature of the heterotype 
division in Ferns (Figs. 16-21). 
1 See also Calkins, 1. c., PI. 295, Fig. 3. 
2 In the contracted condition of the spireme the loops are so closely crowded together that the 
segmentation into chromosomes can only be clearly followed in those loops which project from 
the central mass. 
