NATURE 
[FEBRUARY 4, 1904 
a0) 
320 
recurring changes become visible in the nucleus. 
Delicate strands make their appearance, and these 
finally segment into a number of rod-like or V-shaped 
bodies termed chromosomes. The number of chromo- 
somes thus appearing in any nucleus of the ordinary 
body or somatic cells is quite constant for any species 
of animal or plant, though different species possess 
different numbers of these chromosomes. In man, for 
example, there are thirty-two, whilst in a lily there are 
Fic. 1.—Normal Somatic Mitosis. Fic. 2.—Heterotypical Mitosis. 
twenty-four in all the body- or somatic-cells. The chro- 
mosomes become arranged in a very definite manner 
on a spindle-like structure, and then each of them 
divides longitudinally into two halves (Fig. 1). The 
two halves then separate, each travelling to oppo- 
site poles, and furnish the material out of which the 
chromosomes of the two daughter nuclei are con- 
stituted. 
Although the process, as thus given in the barest out- 
line, is constantly met with in all the somatic cells and | 
may be traced back to the earliest divisions in the fer- 
tilised egg from which the individual has sprung, a 
stage sooner or later is reached in the life-history when 
certain cells become more or less sharply delimited from 
their fellows, and they finally undergo a nuclear divi- 
sion which is very different in character from that met 
with in the other cells of the body. To this particular | 
mitosis the term Heterotype has been applied, and its | 
onset marks a radical change that affects the organisa- | 
tion of the descendants of every cell that has passed 
through it. 
The features by which the heterotype can be dis- 
tinguished from all other mitoses are as follows :— 
The nuclei grow to a relatively larger size, and the 
strands from which the chromosomes arise exhibit a 
very characteristic ‘‘ bunched ’’ appearance at a par- 
ticular stage in the process. Furthermore, the chromo- 
somes only appear in half the numbers characteristic of 
the somatic nuclei. In man, for example, where the | 
somatic number of chromosomes is thirty-two, only 
sixteen appear in the heterotype division. “This reduc- 
tion is due to the cohesion in pairs of the normal 
chromosomes and not to any elimination of them from 
the nucleus. The heterotype chromosomes further | 
differ from the somatic ones in form, and this dif- | 
ference is equally marked in both animals and plants. 
They present the form of rings, loops, &c. (Fig. 2), in- | 
stead of the familiar V-shaped figures; and, further- | 
more, when arrayed on the spindle each divides, ‘not | 
longitudinally but transversely. 
Whilst this is not the place to discuss the significance 
of the remarkable peculiarities that distinguish this 
heterotype mitosis, it is essential to realise that it marks 
the point at which the somatic and reproductive ele- 
ments diverge from each other in their future structure 
and development. At each succeeding division all the 
descendants of a cell that has once divided hetero- 
typically retain the reduced number of chromosomes, 
but in other respects the normal somatic mitoses are 
No. 1788, voL. 69] 
closely simulated (Fig. 3). These  post-heterotype 
mitoses are all distinguished as homotype. 
Thus the appearance of the heterotype mitosis marks 
the definite segregation of a sexual series of cell 
generations. These may be few as in animals, where, 
after a single homotype division, the sexual elements 
are at once differentiated. | In plants, on the other 
hand, it commonly happens that all the descendants of 
the heterotype generation do not actually become dif- 
ferentiated into sexual cells, and in any case the latter 
are only formed after the occurrence of a number of 
intervening post-heterotype divisions. These, how- 
ever, are all characterised by the reduced number of 
chromosomes (homotype), which, as in animals, is 
similarly retained in the nuclei of the sexual cells. It 
is only on the union of ovum and sperm in fertilisation 
that the full somatic number is restored. 
It is perhaps unnecessary to insist that the hetero- 
type mitosis and its consequences are restricted to the 
reproductive tissue, at least, in the normal body; the 
somatic cells of the latter, in so far as they continue to 
divide, present the same features as before. , 
The general bearing of the foregoing description will 
become evident when it is stated that both the hetero- 
type and homotype mitoses have been, during the recent 
investigations, recognised as occurring in certain cells 
of malignant growths. 
If the advancing edge of an actively enlarging 
tumour, such as an epithelioma, be examined, many 
cells will be found to be in various stages of division. 
Near the margin the nuclei commonly exhibit mitoses 
typical of somatic cells (Fig. 1), whilst others will be 
encountered that show irregularities of various kinds. 
An excessive number of chromosomes is not uncom- 
mon (Fig. 4), and here and there pluripolar figures (Fig. 
5) of a remarkable character may be observed. The latter 
occur somewhat unevenly distributed and owe their 
Fic. 3. -Homotypical Mitosis. Fic. 4.—Somatic Mitosis, Polar View. 
Pluripolar Mitosis. 
Fic. 5- 
| origin, at least in part, to the simultaneous division of 
a group of adjacent nuclei on a common spindle appa- 
ratus. Yet other cells will be met with in which the 
process of nuclear division is of a type less complex than 
the normal, and it may be so reduced as to consist in 
the mere drawing apart of the original nucleus into 
two similar or even unequal halves, with a more or 
less complete absence of all differentiation of chromo- 
somes. 
