102 



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, Ascaris niegalocephala, where the facts also 

 seem to be beyond contradiction, Brauer has shown that the tetrads 

 arise by double longitudinal division of the spireme-segments and 

 that no transverse division takes place. In this case reduction is 

 purely quantitative and not qualitative. 



The botanists Guignard (1891) and Strasburger (1888) 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. 



I. Observations. 



Guignard and Strasburger found that in Liliuvi and in Allium 

 the pollen-grain after reduction, undergoes subsequent mitoses, 

 in each of which the same reduced number of chromosomes is re- 

 tained. This led Overton (1892) to suggest that reduction in the 

 higher cryptogams, where sexual and asexual generations alternate, 

 might take place as far back as the formation of the spore. He also 

 suggested that all of the cells of the sexual generation might have 

 the reduced number of chromosomes, and in 1893 he strength- 



