450 Gregory . — Spore- Formation in Leptosporangiate Ferns. 
a qualitative reduction in plants as well as in animals is extremely 
important, as affording a possible provision for that purity of the 
gametes in respect of allelomorphic characters, which is demanded by 
Mendel’s hypothesis. 
There is strong evidence in support of the theory which infers a corre- 
spondence between the development of certain characters in the soma of 
the zygote and the presence of certain chromosomes or groups of chromo- 
somes in its nuclei. 
Boveri’s experiments upon multiple fertilization of the eggs of Sea 
Urchins 1 have shown that the presence of certain combinations of chromo- 
somes is essential to normal development. This direct evidence of 
a qualitative differentiation among the chromosomes is supported indirectly 
by the evidence of the individuality of the chromosomes derived from 
cytology. Of the latter it is only necessary to mention here the work of 
Sutton upon Brachystola magna , in which there is morphological differentia- 
tion (in size) between the chromosomes. 
The chromosome group of the presynaptic germ-cells was shown to 
consist of two equivalent series of chromosomes. In synapsis the homo- 
logous members of the two series fuse with one another in pairs ; at the 
reduction-division the chromosomes which have fused together are relegated 
to different germ-cells. ‘ We are virtually able to recognize each chromo- 
some in eleven consecutive cell-generations. . . . No continuous spireme is 
formed ; and although after each division there is a brief interval, during 
which chromosomic boundaries can no longer be traced, the regular 
correspondence, unit for unit, of the mother-series with the daughter-series 
established a high probability that we are dealing with morphologically 
distinct individuals V 
‘If, as the facts in Brachystola so strongly suggest, the chromosomes 
are persistent individuals in the sense that each bears a genetic relation 
to one only of the previous generation, the probability must be accepted 
that each represents the same qualities as its parent element V 
Wager 4 has recently suggested that the part played by the nucleolus 
during mitosis has hitherto received insufficient recognition. 
He says : ‘ We have in Phaseolus a phenomenon which, if found to be 
a widely spread one, must modify our conception of the significance of the 
chromosomes and nucleolus in heredity 5 . . . . The nucleolus as well as the 
chromosomes will have to be taken into account in any new hypothesis 
which may be put forward 6 .’ 
In the spore-mother-cells of the Ferns 7 the nucleolus is smaller 
relatively to the size of the nucleus than in many plants, and is therefore 
1 Boveri, ’ 02 . 2 Sutton, ’ 02 , p. 34. 3 1 . c., p. 39. 
* Wager, ’ 04 . 5 1 . c., p. 50. 6 1 . c., p. 53. 
7 These observations apply also to Osmunda rcgalis as well as to the Ferns mentioned on p. 445. 
