Philosophy of Biology

Genetic Analysis 259

and further interpreted August Weismann’s hypotheses of the central role of the
cellular nucleus, and especially its chromosomes, in development and differentia-
tion [Allen, 1966, 1978; Griesemer, 2000]. Considering the dispute of embryologists
between preformationists and epigenesists, Wilson concluded that “in some mea-
sure, a reconciliation between the extremes of both the rival theories” should be
found;

[W]e may consistently hold with Driesch that the prospective value of

a cell may be a function of its location, and at the same time hold

with Roux that the cell has, in some measure, an independent power

of self-determination due to its inherent specific structure. [Wilson,

1893/1986, 78]

Since a correlation between sex and the pattern of the chromosomes, as they are
revealed at cell division (mitosis), was found in many animal species, sex differen-
tiation became a favorable subject for the elucidation of the role of chromosomes
in heredity and development. In the cells of sexually reproducing species chromo-
somes may usually be arranged in a species-specific pattern ofpairs — they are
diploid. Following a sequence of two complex cells divisions with only one chromo-
some replication (meiosis) in gametogenesis, only one partner of each chromosome
pair of somatic cell is found in the gametes. Thus upon fertilization of an egg by
a sperm, the species-specific chromosome number and pattern, is restored. The
main exception to this is that of the sex-chromosomes. Whereas as a rule, females
carry a pair of X-chromosomes, in the males only one X-chromosome is present;
the single X-chromosome has no partner or one that differs from it in size or form
— the Y-chromosome (in some groups, like birds and Lepidoptera the femails are
the ones that carry the single X-like chromosome).
The need for a full complement of chromosomes for normal development was
demonstrated in experiments with fertilized eggs (zygotes) of sea urchin (Paracen-
trotus lividus) with excessive or missing numbers of chromosomes that developed
abnormally [Boveri, 1902]. The individuality of chromosomes and their causal
involvement in differentiation was indicated further by the specific abnormalities
of progeny with specific excessive or missing chromosomes.^1 At the same time,
Sutton, one of Wilson’s students, showed that all chromosomes of the grasshopper
Brachystola magnacan be individually identified and that at meiosis the segregat-
ing chromosomes behaved as if each pair were alleles of a Mendelian factor [Sutton,
1903]. Sutton pointed out that if each pair segregated independently of the others,
different combinations of the chromosomes (12 pairs in females) would produce a
very large number of patterns (2^12 = 4096), as expected of the Mendelian theory
of variation.

(^1) Twenty years later, Blakeslee [1922] succeeded in his hybridization experiments with the
Jimson weedDatura stramonium, to differentiate the phenotype of the fruiting-bodies of all
possible 12 trisomics — i.e., otherwise diploid plants with one of the chromosomes represented
three times instead of twice — correlating each to the specific chromosome that was excessive.