1911] GATES—CHROMOSOME REDUCTION 333 
parallel threads, and (3) because even if there were such units 
they would have an equally good or even better opportunity for 
interchange during the alveolated reticulum stage which always 
intervenes between somatic mitoses. 
From this point of view, the life cycle of any sexual plant or 
animal (with reservations for the Ascomycetes and other groups 
where peculiar conditions of sexuality occur) may be outlined as 
follows: At or soon after fertilization, the materials composing 
the sets of chromosomes of the egg and sperm nuclei become 
arranged in pairs, so that in subsequent mitoses throughout the 
sporophyte or soma they always reappear as pairs of homologous 
chromosomes, the members of which originated respectively from 
the egg and the sperm or male cell. Synapsis plays no special 
part in the pairing, and indeed appears occasionally to be omitted 
in some forms.’ Meiosis or reduction consists essentially in the 
Segregation of the members of these pairs which have been in 
association since soon after fertilization. This segregation is 
followed immediately by what is essentially another mitosis. 
The gametophytic or germ cell nuclei may then continue to divide 
With the haploid number of chromosomes until the diploid number 
is restored by fertilization. The chromosomes are therefore in 
pairs from the time of fertilization onward, and the members of 
the pairs are merely segregated in the heterotypic mitosis. The 
Completion of the act of fertilization is not deferred until synapsis, 
but takes place probably soon after the union of the sexual nuclei; 
and throughout the sporophyte or soma the chromosomes main- 
tain, to some extent, their relative space relations with each other. 
This leaves several obvious points unexplained. Why are 
there almost universally ‘wo meiotic divisions without a growth 
Period of the chromosomes between them? The conception of 
* Gr&corre (12, p. 332) says regarding synapsis: “Le ramassement synaptique 
ne peut avoir, par lui-méme, aucun réle a jouer dans |’accomplissement des phé- 
noménes de réduction, mais doit étre considéré plutét comme une conséquence des 
Phénoménes essentiels qui se déroulent dans le noyau. Cela résulte de ce que, dans 
Certains objets, on ne retrouve pas le ramassement synaptique (SCHREINER, JANSSENS, 
Deron) et que néanmoins les stades leptoténes, pachyténes (et méme zygoténes) 
Y montrent une évolution absoluement identique 4 celle que l’on constate dans les 
objets od se manifeste un ramassement.” 
