446
T . C . HSU
heterochromatin may be A - T polymers and others G-C polymers
seems interesting, although no supporting experimental data were
given. One observational fact may favor this theory, namely, the
non-specific pairings of heterochromatin during meiosis and in salivary
glands. This may also explain the terminal associations consistently
noted among the acrocentric chromosomes of man, for, according to
the idea of Hsu and Somers, telomeres may also be A - T polymers.
Utilization of an agent which is known to deaminate cytosine, hydroxylamine (Freese, Bautz-Freese and Bautz, 1961), has enabled Somers and
Hsu (1962) to locate specific constricted areas and breakages which
differed in locations from those induced by B U D R . O f particular
interest is that the centromere regions are susceptible to hydroxylamine attack. It is not absurd to think that the areas responding to
hydroxylamine damage may contain a high content of G-C pairs.
Perhaps I am over-optimistic, but it does appear likely that with the
advances in concepts and in methods, more knowledge will be gained
in the not too distant future concerning the nature and function of
heterochromatin.
V . S E X C H R O M A T I N , S E X C H R O M O S O M E S ,
S E X D E T E R M I N A T I O N
The primary motive for studying cells and tissues in vitro, perhaps
ever since Harrison's first experiment, is to gain information for the
understanding of cells and tissues in vivo. It is probably satisfying to say
that in the fields of genetic cytology and cell biology, cell culture has
contributed a great deal toward this ultimate goal. In this last section
we shall endeavour to make an integrated effort, from what we have
presented previously in studies on in vitro cells and some information collected from in vivo studies, to discuss what we have on hand pertaining
to the problems of sex chromatin, sex chromosomes, and sex determination in mammals and man.
As mentioned, sex chromatin in female cells was regarded as the
fused heterochromatic parts of the two X chromosomes, implying
somatic pairing of the X elements. One argument against this hypothesis is that if both the X chromosomes contribute to the formation
of a sex chromatin, the male cell containing a single X should exhibit
a smaller sex chromatin also. This is not the case. In fact, even the
correlation between the sex chromatin and the sex chromosomes was
based largely upon circumstantial evidence. Recently, however, cytological data favor the notion that sex chromatin in female cells represent a single X chromosome (Grumbach andMorishima, 1962). Among
T . C . HSU
heterochromatin may be A - T polymers and others G-C polymers
seems interesting, although no supporting experimental data were
given. One observational fact may favor this theory, namely, the
non-specific pairings of heterochromatin during meiosis and in salivary
glands. This may also explain the terminal associations consistently
noted among the acrocentric chromosomes of man, for, according to
the idea of Hsu and Somers, telomeres may also be A - T polymers.
Utilization of an agent which is known to deaminate cytosine, hydroxylamine (Freese, Bautz-Freese and Bautz, 1961), has enabled Somers and
Hsu (1962) to locate specific constricted areas and breakages which
differed in locations from those induced by B U D R . O f particular
interest is that the centromere regions are susceptible to hydroxylamine attack. It is not absurd to think that the areas responding to
hydroxylamine damage may contain a high content of G-C pairs.
Perhaps I am over-optimistic, but it does appear likely that with the
advances in concepts and in methods, more knowledge will be gained
in the not too distant future concerning the nature and function of
heterochromatin.
V . S E X C H R O M A T I N , S E X C H R O M O S O M E S ,
S E X D E T E R M I N A T I O N
The primary motive for studying cells and tissues in vitro, perhaps
ever since Harrison's first experiment, is to gain information for the
understanding of cells and tissues in vivo. It is probably satisfying to say
that in the fields of genetic cytology and cell biology, cell culture has
contributed a great deal toward this ultimate goal. In this last section
we shall endeavour to make an integrated effort, from what we have
presented previously in studies on in vitro cells and some information collected from in vivo studies, to discuss what we have on hand pertaining
to the problems of sex chromatin, sex chromosomes, and sex determination in mammals and man.
As mentioned, sex chromatin in female cells was regarded as the
fused heterochromatic parts of the two X chromosomes, implying
somatic pairing of the X elements. One argument against this hypothesis is that if both the X chromosomes contribute to the formation
of a sex chromatin, the male cell containing a single X should exhibit
a smaller sex chromatin also. This is not the case. In fact, even the
correlation between the sex chromatin and the sex chromosomes was
based largely upon circumstantial evidence. Recently, however, cytological data favor the notion that sex chromatin in female cells represent a single X chromosome (Grumbach andMorishima, 1962). Among