CHROMATIN-REDUCTION AND TETRAD-FORMATION IN PTERIDOPHYTES.
From the time when Van Beneden, in 1883, found that mature reproductive cells have only half as many chromosomes as the ordinary somatic cells, until the present time, cytologists have endeavored to explain the meaning of this reduction and to show how it takes place. These efforts have not as yet been attended with complete success.
One of the most widely known theories as to the meaning of reduction is that of Weismann who accepted the earlier concep tion of Roux (1883) as to the significance of mitosis and built upon it an elaborate theory of development. In this he predicted that a form of mitosis would be found in the maturation of the reproductive elements " in which the primary equatorial loops are not split longitudinally " (p. 371) and " by means of which each daughter nucleus receives only half the number of ancestral germ-plasms possessed by the mother nucleus" (p. 375).
Weismann's prediction has been confirmed by recent observa tions on the copepod crustacea, and it is now known that in this group of animals at least, the chromosomes of a maturing cell un dergo a transverse division, giving reduction in the Weismann sense. This process of reduction, wherever definitely made out, is invariably preceded by an arrangement of the chromatin into four-parted chromosomes, to which the name " Vierogruppen" or " tetrads " has been given. These tetrads are always half as nu merous as the chromosomes of the somatic cells and often differ widely from them both in shape and size.
The entire question of reduction rests upon the manner of tetrad-formation, but unfortunately, observers are diametrically opposed in their descriptions of the process. On the one hand it
has been shown beyond question, that in some cases (the copepod crustacea) two of the four parts of the tetrad are formed by longi tudinal division of the spireme-segment, while the other two arise by transverse division. In such cases two successive mitoses divide the tetrads, first into two dyads and second into single elements. By these two divisions the resultant reproductive cell receives one-fourth of each of the original tetrads. On the other hand, in another case, ASCaliS lllegt7/0Cepht71(1, where the facts also seem to he beyond contradiction, Brauer has shown that the tetrads arise by double longitudinal division of the spiretne-segments and that no transverse division takes place. In this case reduction is purely quantitative and not qualitative.
The botanists Guignard (1891) and Strasburger (i888) have maintained that in plants also, a reduction in the Weismann sense does not take place. Neither Guignard nor Strasburger found tetrads. They described the spireme as breaking up into half the normal number of chromosomes which undergo simple longitu dinal division at each successive mitosis.
It would be remarkable if a process so general in animal cells as the formation of tetrads should not be found in plant cells, and with the hope of finding some evidence of this in plants I under took the study of reduction in the group of Pteridophytes, the re sults of which are given in the following section.