144 
The Ohio Naturalist. 
[Vol. XIII, No. 7, 
mosomes is larger as is the case in most organisms and each of the 
chromosomes is oriented by chance independently of the rest 
as is presumably the case it is obvious that the number of com¬ 
binations i. e. the number of kinds of reduced cells increases as the 
square of the number of chromosomes. 
Omitting the variations, however, and following one of the 
nonsexual spores, say that with chromosomes marked with 
circles and dots, we find that it produces on germination the 
familiar heart-shaped gametophyte (prothallus) of the fern. 
The mitoses occurring in the growth of this plant (Figs. 14 and IS), 
are exactly similar to those of the sporophvte except that they 
have only the reduced number of chromosomes found in the 
spore from which it grew, i. e. they are haploid instead of diploid. 
When mature the gametophyte produces archegonia bearing 
eggs, and antheridia bearing sperms. In the development and 
maturation of these gametes there is, of course, no reduction 
division. 
Fertilization may occur between an egg and a sperm from the 
same plant or the sperm may come from a different gametophyte. 
The latter alternative is figured in the diagram and it is further 
assumed that the sperm came from a gametophyte derived from 
a spore bearing the chromosomes marked with crosses (Figs. 
20 and 21). When the sperm fuses with the egg their nuclei 
may be in a resting condition or they may be resolved into their 
respective chromosomes (Figs. 19-22). and proceed at once into 
the first mitosis of the succeeding embryo and the cycle is com¬ 
plete. (Figs. 23-25). 
The significance of the conventions adopted in marking the 
chromosomes thus becomes apparent. Those marked with dots 
and circles came from the egg parent and those marked with 
crosses from the sperm parent. In view of this, the fact com¬ 
mented upon above that each chromosome pairs with its ap¬ 
propriate mate in synapsis, takes on a new significance, for each 
of the tetrad or reduction chromosomes is seen to consist of a 
doubled chromosome of maternal origin paired with the cor¬ 
responding one of paternal origin. It is also evident that 
while the nuclei fuse in fertilization, the chromosomes do not 
show any sexual affinity for each other and live together, so to 
speak, in the nuclei of the diploid generation as independent units, 
until in the first half of the reduction division the corresponding 
pairs of maternal and paternal chromosomes appear to develop an 
attraction for one another and finally unite as synaptic mates to 
form the reduction chromosomes, so completing the union of 
sexual elements begun at the time of fertilization. 
It is obvious, moreover, that if by chance one of the chromo¬ 
somes had been oriented differently in the reduction division, as 
indicated by the alternative Figure 10a, none of the spores result- 
