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Recent advances in cytology. By C. D. Darlington. Second edition. Philadelphia. P. Blakiston's son and Co. 1937. xvi + 671 pp

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Second edition. Philadelphia. P. Blakiston’s Son and Co., 1937.
671 pp.
The reader who turns to ‘Recent Advances in Cytology’ with the hope
of finding any extended discussion of cytoplasmic features will be
disappointed. Mitochondria are mentioned once, the Golgi apparatus
not a t all. Of the structures beyond the nuclear membrane only the
centrosome receives extended consideration. There are, it is evident,
specialists even among cytologists, and this treatise is for those who
are interested in the structure and behavior of chromosomes. But
though the field covered is in one sense narrow it is in another sense
broad, for there is a remarkable consistency in the phenomena of
chromosomal behavior throughout a wide range of forms from protozoa
to vertebrates, and the impressive body of basic information that
cytologists have accumulated is in some measure fundamental to all
other phases of biology.
The book is not one to be picked u p casually and read of a n evening
to see what the cytologists have been doing. The style is at times
difficult and the reader may need paper and pencil to clarify some of
the numerous diagrams or to adapt them to his own mode of thinking.
Most readers will profit by having a t hand four lengths of differently
colored twine to represent chromatids (tetrads), with properly spaced
knots which can serve as chromomeres. Scissors and mending material
will also prove useful in elucidating some of the relations in inversions
and chiasmata (‘xta’in the diagrams). I n short, one should approach
this book much as he would a volume on stereochemistry.
During the past 10 years Darlington has contributed some fifty
or more significant papers on cytogenetics and has achieved a
position among the most original and stimulating workers in the field.
Throughout the present discussion there is an apparent inclination to
push the analysis to the limit and, insofar as possible, to interpret
the behavior of cellular elements in terms of chemical and physical
forces at the molecular level. The author’s point of view may perhaps
best be indicated by quoting directly a few sentences from the preface.
“Cytology” he says (‘began by describing what sort of things cells
were. It continued by inferring what things happened inside them.
69,NO. 1
Its last and longest task is to discover why these things happen.
. . . . Finding out why things happen in cells is a n entirely different
matter from finding out how they happen. The one problem is a matter
of skill and common sense. The other takes us into a new element.”
The new element, it would seem, is that in which chemists and physicists
flourish. But even in this medium we shall indeed be fortunate if the
whys of today do not become merely the hows of tomorrow. When they
cease to do so perhaps science will have reached its goal. Be that as it
may, the new orientation in which deductive reasoning is recognized
as having a valid role, is heartily approved by J. B. S. Haldane, who
points out in a n appreciative foreword that “this attitude has long
been normal in chemistry and physics, ” and adds that its introduction
into biology is a sign of the growing unity of science. Indeed Haldane
goes so far as to suggest that “ i t is perfectly possible that ‘Recent
Advances in Cytology’ marks a turning point in the history of
biology. ”
A t the present stage of the analysis it would seem that any account of
how cells divide must take cognizance of centrosomes and centromeres,
remarkable entities which have much in common. The centrosome
lies outside the nucleus ; the centromeres, each of which is in a sense
the very heart of a chromosome, lie within. Surrounded by media
which are not isoelectric with them these bodies, in common with other
cell organs, develop surface charges which are subject to fluctuations
coincident with changes in the pH of cell fluids. Calculations of
Raldane are cited to show that the resultant attractions and repulsions
must be of a magnitude that could account for some of the observed
phenomena of cell division. When the centrosome has divided and
the daughter centrosomes have become oriented a t the poles of the
liquid crystal which transmits electrical forces with
maximum efficiency-the similarly charged centromeres are forced
into a relatively neutral position in the equatorial plane. As the polar
repulsions decline the centromeres divide and then, repelling each
other, drag their respective half chromosomes to opposite poles. In
this process we haw: first a centrosome spindle, then a centrosomecentromere spindle, and finally a centroinere spindle. The apparent
absence of observable centrosomes in the higher plants may mean that
in them the centrosomes are small o r that they do not respond to ordinary fixing and staining methods. Another possibility is that their
function may be taken over by some other element in the cell. Tf the
interpretations are sometimes lacking in complete and rigid proofs,
they are at least based on a definite hypothesis with postulates than can
be tested.
The. general character of the explanation (or description) which
Darlington offers for the main features of mitosis can be inferred from
the account of behavior of centrosome a n d centromeres. A few salient points may be mentioned for contrast with the familiar statements in conventional texts. When the chromosomal threads first
appear a t prophase they are already double. Their visibility at this
stage is probably brought about through dehydration of chromatin
t h a t has been in a dispersed state. There is a t no time a continuous
‘spireme,’ each chromosome (pair of chromatids) being independent
throughout mitosis, as it probably is during resting stages. Shortening of chromosomes which occurs in late prophase is essentially dur to
a process of spiralization, relics of which can be detected in the
partial uncoiling a t telophase. Each chromosome has a centric constriction marking the site of the centromere or ‘spindle fiber attachment,’ a n d frequently secondary constrictions which indicate
the position of ‘nucleolar organizers.’ A t metaphase the centromere
is always in or on the spindle but arms of longer chromosomes are
free t o wave t o and fro in the current of streaming cytoplasm.
Despite occasional appearances to the contrary, division of the
chromosome, or better separation of chromatids, always begins at
the centromere and progresses toward the distal ends of the arms
as the daughter centromeres push apart. The apparent size of a
chromosome, perhaps determined in some degree by the tightness
of coiling, may be influenced by factors external t o itself, a s shown
in properly selected hybrids.
It is in dealing with meiosis, which is the main theme of the book,
that the extent of recent advances and the wealth of authenticated data
are most impressively presented. Here, too, interpretive ingenuity is
afforded ample scope. The treatment is critical, and to a degree
historical, but scant notice is taken of alternative views or ideas
that are thought to have been superseded. According to Darlington
“the difference between meiosis and mitosis consists in the external
mechanism which acts twice getting out of step with the internal
mechanism which acts once. . . . . The two forms of division are
distinguished by a simple physiological difference. ”
In the prophase of meiosis the chromatin threads that appear are
not double and are 2n in number. Polarized (bouquet stage) or not,
the homologues unite in pairs, chromomere by chromomere, beginning
usually, but not necessarily a t the centromere. There may be terminal
or intercalary segments in which union does not take place a t all.
This is particularly likely to be the case in long chromosomes where
division into chromatids if initiated before synapsis is complete may
inhibit further lateral fusion. I n this connection the time element is
a n important one. During the early phases of meiosis homologous
chromosomes are wound around each other-‘ relationally coiled’-and
the pairs of chromatids into which each divides show a similar coiling,
but in the opposite direction. The origin of chromatids, however,
results in a readjustment of internal attractions so that the two pairs
tend t o fall apart. But in doing so they produce an increased tension
in the longitudinal direction which is frequently sufficient to snap
one of the strands, the two ends of which can then partially uncoil.
This creates a still greater stress in the other pair and one of them
also breaks a t the same place. The four free ends now make new unions
by virtue of their terminal attractions and ‘crossing-over ’ is thus
effected. It follows that the two chromatids that may sometimes be
seen to lie across each other in a chiasma are not the two that broke.
After having been formed a chiasma may move along (like a noose)
toward the end of an arm to become terminalized, but in doing so it
may not cross the centromere. The formation of chiasmata and thc
processes involved in their terminalization are responsible for many
pages that require close application on the part of the reader. It may
be inferred that the Belling hypothesis is no longer tenable. Similarly
the distinction of equational and reductional divisions in meiosis
has been relegated to a position of merely historical interest, since
the chromatids that separate following chiasmata formation represent a niosaic of materials that entered a t synapsis and consequently
some pairs of homologous genes are separated a t the first division
and others a t the second.
It would be misleading to leave the impression that the volume is
limited to a consideration of the mechanics of cell division and analysis
of intercellular forces. There is a n extended discussion of the
nature and genetic implications of ring formations, and consideration
of the origin and behavior of polysomics and polyploids, creation of
new species, changes in chromosome complements, evolution of sex,
hybrid vigor and sterility. Most of these subjects are treated with
considerable adequacy, and if at times the treatment leaves a lingering
doubt it never fails to be thought provoking. On the whole the book
is rather heavily weighted on the side of plant cytology, but justly
so for work has been most active in that field. It may be due t o a
subconscious botanical bias that table 12 relating to a number of
plants and some fifty animals is labeled “Provisjonal Classification
of Annuals and Plants.’’
The book concludes with four appendices : a provocative resum6
entitled Interpretatioa; three pages of references o n cytological technique ; a glossary of many, but not all, of the special terms employed ;
and a bibliography of some 1700 titles.
Stanford University
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edition, recen, second, 1937, blakistoni, advanced, 671, darlington, xvi, son, philadelphia, cytology
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