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LECTURES ON SEX AND HEREDITY

MACMILLAN AND CO., LIMITED

LONDON • BOMBAY • CALCUTTA • MADRAS
MELBOURNE

THE MACMILLAN COMPANY

NEW YORK • BOSTON • CHICAGO
DALLAS • SAN FRANCISCO

THE MACMILLAN CO. OF CANADA, LTD.

TORONTO

LECTURES ON
SEX AND HEREDITY

DELIVERED IN GLASGOW, 1917-18

BY

F. O. BOWER
J. GRAHAM KERR

AND

W. E. AGAR

MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON

1919

COPYRIGHT

GLASGOW : PKINTKI) AT THK I'MVKKSITV I'KI-ss
l:Y kOUERT MACLEHOSE AND CO. LTD.

CONTENTS

PAGE

INTRODUCTORY ... i

LECTURE I
THE ORIGIN OF SEX IN PLANTS

LECTURE II

THE EFFECT OF A FIXED POSITION ON THE SEXUALITY

OF PLANTS 32

LECTURE III

THE REPRODUCTIVE PROCESS IN ANIMALS : SOME OF THE

GENERAL PRINCIPLES 51

•

LECTURE IV

SOME OF THE MODIFICATIONS OF THE REPRODUCTIVE

PROCESS AS ADAPTATIONS TO LIFE UPON LAND - 66

vi CONTENTS

LECTURE V

PAGE

HEREDITY - - 81

LECTURE VI
HEREDITY IN MAN 100

GLOSSARY AND INDEX - - - - 1 1 q

INTRODUCTORY

\

THE object with which these Lectures were given was to
convey in as simple terms as possible the leading facts
relating to Sex in Animals and Plants, together with
suggestions bearing on the use and effect of sexual
propagation. Sex is a very wide-spread fact among
living things. Its manifestations are most obvious in
the Higher Animals and in the Higher Plants. But
sex is not general for all living organisms. Some very
simple and, as we believe, primitive beings are sexless,
while others show rudimentary sexual characters, others
again more advanced conditions. It is possible to arrange
such examples in order, so as to show how the sexual
mode of propagation may have come into existence, and
how in the rising scale of Animals and Plants the differ-
ence of the sexes became gradually more marked. This
is the evolutionary aspect of the theme, and it will be
taken up in the two first Lectures as it is seen in Plants.
The third and fourth Lectures will deal with the same
question in Animals. A discussion follows in the fifth
and sixth Lectures on Heredity.

The whole body of any one of the higher Animals or
Plants is made up of various tissues, differing in texture
and in function. In the Animal body everyone recognizes
skin, muscle, bone, and nerve. In the Plant body
there is also an external skin ; tissues that are soft and
sappy when young lie within it, together with firmer

S.H A

2 SEX AND HEREDITY

strands that mature into wood and bast in older trunks.
Such tissue-masses are familiar to everyone. Their micro-
scopic study shows that, whether in Animals or Plants,
these tissues are all made up of certain structural units,
called cells. Each cell is so minute that as a rule it
cannot be seen with the naked eye. It consists essen-
tially of a viscous body of protoplasm, a substance of the
nature of white-of-egg, in which all the activities of Life
are carried out. Within it lies a definite body of roundish
or oval form, the nucleus, which is itself part of the Proto-
plasm. It appears to dominate the cell, and serves as
the centre of its vitality. Very simple and minute
organisms may consist of only one cell each. These are
called unicellular organisms, and they lie at the base of
the series either of Animals or Plants. But those Animals
or Plants which are ordinarily known as such by the
general public are multicellular . They are composed of
very many, even of millions of these structural units, or
cells, which live together a common life, and compose
what is known as an individual, such as a horse or a tree.
We may go further and say that the whole of each Animal
or Plant is composed of such cells, or their derivatives. This
has been called the cell-theory. But it is now so fully
proved in detail that it may be stated not as a theory but
as a fact of observation (Fig. i).

There is reason to believe that Animals and Plants have
advanced independently in the origin of Sex. But not-
withstanding this, one of the most striking features in
the whole story is that the leading facts of sexuality in
Plants and Animals are very much alike. Their similarity
is indeed so great that the same terms may be used in
describing both. The essential feature of Sexuality
whether in Animals or Plants, consists in the fusion of two

INTRODUCTORY 3

cells, derived from more or less distinct sources, to form one
new cell. The parental cells which thus fuse are called
sexual cells, or gametes, which are detached from the
parental body. The result of their fusion is called a

^^M^^Ai^^ v- i irx

A

. ..•V.:::-'u '?.">*> .

_/:•:.- •••••

FIG. i.

A, Five young vegetable cells, each with its granular protoplasm surrounding
a large spherical nucleus. Each cell is delimited by a cell-wall which is thin while
young, but may become much thickened as the cell becomes mature. ( x 800.)

B, Part of a transverse section of the stratified, epithelial lining of a cat's
oesophagus, b.m., basement membrane. Here each cell is not delimited by a cell-
wall, as is the case in plant-tissues. ( x 700. From Dahlgren and Kepner.)

zygote, and it forms the starting point for the develop-
ment of a new individual. This individual may remain
unicellular, as in the case of the simplest living things.
Or, as in higher organisms, it may undergo cell-division ;
and this if continued may give rise to a large and complex
individual, such as a horse or a tree. But whatever the
size or form of the sexually-produced offspring may be,
the genesis of it is essentially the same, by fusion of two

4 SEX AND HEREDITY

sexual cells, which are developed for the purpose. This
fusion is called " Syngamy."

In very simple sexually produced organisms, whether
Animals or Plants, these parental cells are both alike.
They are then called Isogametes, and in most primitive
organisms they are motile through water. Such gametes
are believed to represent a rudimentary and primitive
state. A more advanced type is that where the two are
unlike. The difference is first seen in size, and this is
usually followed also by difference in motility. The
smaller is as a rule more actively motile, and it is called
the MALE GAMETE, or micro- gamete, or spermatozoon, or
spermatozoid. The larger is as a rule non-motile, and is
called the FEMALE GAMETE, or macro-gamete, or ovum, that
is the egg. All these terms are equally applicable to the
phenomena shown in Animals and in Plants, and they
will be used in the descriptions which follow (Fig. 2).

The most obvious result of sex, as seen in the higher
Animals and in many of the higher Plants, appears to be
an increase in the number of individuals. In horses, in
cattle, and in Man this is the only method of propagation.
The same is the case with some Plants, such as the Pines
and Firs, and certain large brown Seaweeds. But while
this is true for the most advanced Animals, and for some
highly organised Plants, it does not apply for organisms
lower in the scale. In these, whether Animals or Plants,
there are other methods of increase in numbers, for
instance by various types of budding. Such budding is
very common indeed among the Highest Plants. It
would then be a mistake to think that for living things
generally multiplication is only by a sexual process, or
that multiplication and sexuality are necessarily con-
nected. A study of the lower organisms shows clearly

INTRODUCTORY

••;v-:^/>.'5-- \;i->:-T ';:* -'.-V^.V . "'-.: ' • •- •"

; •

• ' •:• -, ^-U • ..; &g£

••' .'.•» *»•>• .' -*i ' -' /--*."-•• »C... •*--..>•« ,*,*,

FIG. 3.

^4, Large spherical egg (female gamete) of Bladder Wrack (Fucus), surrounded
by many small motile spermatozoids (male gametes). (After Thuret. x 330.)

B, Human gametes highly magnified : egg to the right, spermatozoon to the
left. The egg is enclosed in a transparent envelope which disappears during
development. Each division of the scale represents .05 mm., or about 7-,1,,, inch.

6 SEX AND HEREDITY

that increase in numbers was not in the first instance
an essential consequence of sex. In the very simplest
Animals and Plants sex involves an actual decrease in
number. Where the whole organism consists of only a
single cell, and two such individuals fuse sexually to form
one zygote, obviously the immediate result is a fall in
number to one half. So we must seek some other initial
reason for sex than increase in number. Many believe
that in the first instance the advantage following from
sexual fusion lay in nutrition. The two gametes fused
together form a stronger cell than either of them was
alone. It seems natural then to conclude that in uni-
cellular organisms sexuality may have originated in the
nutritional advantage that followed on their fusion.

But with this fusion there follows as a consequence
the pooling of such qualities as the fusing cells them-
selves possess. So far as these qualities can be trans-
mitted to the offspring, the mechanism of fusion offers
the opportunity for their transmission. And we do
positively know that the general characteristics of the
parents, and even many quite trivial peculiarities, are
liable to be transmitted to the offspring. Any human
family gives evidence of this. What is seen in Man
appears also in Animals and Plants, as every breeder of
prize stock and every horticulturalist knows. There is
no doubt whatever that Heredity is a fact. But the
chances that the sexually produced offspring will exactly
repeat all the characters of either parent are extremely
remote. It shares the characters of both parents, but practi-
cally in all cases it differs in some degree from each of them.

Whether produced, as in very simple creatures, by
direct fission, or as in more advanced organisms by
budding, or through sexual fusion, the progeny is always

INTRODUCTORY 7

derived from a living source. There is no such thing as
spontaneous generation known to scientific men. Life
under present conditions is always derived from life.
The processes of fission or of budding suggest merely the
increase in number of living things without any change
in quality ; and in practice the offspring of budding
is found to repeat, as a rule, very exactly, the characters
of the original organism from which the bud sprang.
In Plants this fact is made use of by horticulturalists.
Roses are propagated by 'budding"; strawberries by
' runners " ; apples, pears and plums by " grafts," which
are each of them parts of an original plant removed and
encouraged to grow on as new individuals. These retain
the exact qualities of colour, scent, and flavour of the
original stock, though not necessarily the size, or season
of maturity. But it is different with the offspring pro-
duced by the sexual method. The fact that it is a blend
of two parental characters at once distinguishes it from the
result of mere fission or budding. The sexually produced
offspring is not a mere repetition of either parent. In the
fusion of gametes there is a mechanism which provides
for a summation of parental characters from both parents.
But it is a matter of common observation that the sexually
produced offspring docs not repeat all the characters of both
parents. The later Lectures of the series will take up the
question how the characters that are inherited are dis-
tributed in the sexually produced offspring. And as many
of the Higher Plants and all of the Higher Animals,
including Man himself, are sexually produced, it will be at
once seen how large a question is here involved. For
sexual propagation is not a mere matter of increase in
number, but one which touches the very springs of
Evolution and of Progress in Living Beings.

LECTURE I

THE ORIGIN OF SEX IN PLANTS

DIFFERENCES of Sex, and the fact that the sexes mate
and breed, are obvious for the Higher Animals, and for
Man himself. Hence the fact of Sex in Animals was
recognised from the earliest times. This is shown by the
Mosaic Cosmogeny, which reflects the knowledge of still
earlier periods. There is, however, no such early reference
to Sex in Plants. It is true Aristotle and Theophrastus
explained certain facts relating to the Fig and the Date
on a theory of Sex, based on an assumed analogy with
what was then known of the Higher Animals. But this
was no more than vague surmise. Real knowledge of
the facts of Sex in Plants was still wanting, however
clearly it might be realised that propagation was effected
by means of the Flower, and the Seeds produced from it
in the Higher Plants.

Neither in Animals nor in Plants could any detailed
knowledge of Sex be expected till after the invention of
the compound microscope, for the sexual cells themselves
are minute. The compound microscope came into use
at the end of the seventeenth century. But it was long
after that before the details of sexual propagation in
Plants were actually observed. The story has been

8

THE ORIGIN OF SEX IN PLANTS 9

rapidly advanced in the last decades. Many Botanists
still living have witnessed the gradual steps of observa-
tion. These were first made among the simpler, or lower
organisms. Observations have since been extended, till

FIG. 3.

Euglena gracilis. A, form with green colouring bodies (ch.) ; n = nucleus ; v =
vacuole and red eye-spot; g = flagellum; 5 = form with small green colouring
bodies; C = colourless saprophytic form, occurring in nutrient solutions in
absence of light ; D — resting cyst ; r = red eye-spot ; E = germination of resting
cyst by division into four daughter cells, which later escape. (After Zumstein.
A, C x 630 ; B x 650 ; D, E x 1000. From Strasburger.)

now a continuous record can be given even of the
complicated sexual production of new germs in the
Higher Plants. Moreover, by comparison of lower and
simpler forms of Plant-Life it is possible now to obtain
some idea of the origin and nature of Sex in Plants. A

IO

SEX AND HEREDITY

,-m

few examples will be described illustrating the probable
course of its Evolution.

The first example is one of those lowly organisms
which it is difficult to rank definitely either as an Animal
or as a Plant ; for it shares the qualities of both. It is
Euglena, one of the Flagellates, which in summer is
commonly found colouring the foul water draining from
manure heaps a vivid green. If a drop of this water be
examined under the microscope, many free-swimming,
pear-shaped bodies will be seen, propelled each by a

single lashing flagellum
(Fig. 3). Each one is a
separate individual, and
contains within its little
naked mass of protoplasm
a nucleus, and several
green bodies to which its
colour is due. There is
also a bright red eye-spot
close to the base of the
flagellum or cilium. These
individuals multiply by fission or cleavage of the motile
cell to form two equal parts, each of which is a new indi-
vidual (Fig. 4). Here then is an increase in number
without any sexual process. The organism is motile in
water, a characteristic shared by many very simple
Animals. There is, however, a second phase of its life
which it enters when the circumstances are unfavourable.
The creature ceases to move, and surrounds itself with a
cell- wall. It becomes encysted (Fig. 3, D). But later,
when the conditions are favourable, it reverts to the
motile state, its protoplasm dividing, and escaping
from the ruptured cyst. In its encysted state Euglena

FIG. 4.

Successive stages of fission in Euglena : semi-
diagrammatic.

THE ORIGIN OF SEX IN PLANTS

ii

structurally resembles many simple Plants. Whether
in the motile or the encysted phase it shows its primitive
character by being unicellular, and by the absence of
any sexual mode of increase.

Another simple example is seen in Protococcus viridis,
which is commonly found growing as a fine green powder
on the windward side of tree-trunks in damp climates.
The single cell is stationary,
spherical, and bounded in a
cell-wall (Fig. 5). In these
respects it resembles the
encysted Euglena. Each
cell may divide, and the
divisions may be repeated,
so that a group of cells is
formed. But sooner or
later they round off and
separate, each being in fact

an independent individual. - ^

Here again no sexual pro-
cess is known. In being FlG 5

non-motile, encysted, and Protococcus viridis, a unicellular plant, existing

singly, or in groups. ( x 730.)

containing green chloro-
phyll, Prolococcus shows features that are usual in Plants.
It is accordingly referred to the Plant-Kingdom. These
two examples will serve to suggest that very minute and
simple creatures exist, some of which are difficult to refer
to the one Kingdom or to the other ; and that they show
no sexual process whatever. They are probably very
primitive living beings, such as in Evolution preceded the
introduction of Sex.

The next example is Ulothrix, a small green Alga found
very commonly attached to stones in fresh-water streams.

12

SEX AND HEREDITY

It is not quite so simple in structure as those before des-
cribed, for it consists of a number of encysted cells

FIG. 6.

Uluthrix zonata. A, young filament attached by rhizoid, ;• ( 300) ; B, portion
of filament with escaping swarm-spores ; C = single swarm-spore ; D = formation
and escape of gametes ; E= gametes ; F, G = conjugation of gametes ; flr = zygote;
/ /y^otc alter period of rest; A" = zy;jote aftrr division into swarm-spores.
(After Dodel-Port. From Strasburger. B-K&4&2.)

attached end to end, and sharing a common life. It is
thus a Plant with a multicellular body, which is fixed at
its base to the substratum (Fig. 6). In this state the
plant leads a vegetative existence, its cells nourishing

THE ORIGIN OF SEX IN PLANTS 13

themselves, growing and dividing. But sooner or later
the protoplasmic contents of some of its cells, usually after
division into two or more parts, may escape through a hole
in the cell-wall, as motile naked bodies, into the water in
which the plant lives. In fact they pass from the stationary
encysted state, where each formed part of the fixed plant-
body, to that corresponding to the motile Euglena. Some-
times the whole protoplasm of a cell escapes without
division, but more commonly it divides into two or more
parts. According to the number of divisions, the motile
bodies differ in their size. If the protoplasm is un-
divided, or has divided into a small number of parts such
as two or four, these escape into the water through an
opening in the wall, as relatively large, pear-shaped zoo-
spores ; this name is given to them because they show
active movements, due to the lashing of four delicate
cilia attached to the transparent pointed end (Fig. 6, C).
Each zoo-spore consists of a small mass of living proto-
plasm, with a nucleus, a green chlorophyll-body, and a
red eye-spot ; features which it shares with Euglena,
though differing in the number of the cilia. After a
period of movement in water it settles, draws in its cilia,
and forms a cell-wall. This encysted cell may then
germinate, and divide to form a new multicellular filament
like the parent. This is a vegetative or non-sexual mode
of increase, which merely secures multiplication, together
with dispersal of the new plants by the movement of the
zoo-spores.

But in other cases the division of the protoplasm of
the cells of the Ulothrix-fLlameut may be carried further ;
consequently the parts will themselves be smaller, though
more numerous ; and when these escape as before, they
appear similar in form to the zoo-spores, but only with

14 SEX AND HEREDITY

two cilia (Fig. 6, D). They are gametes, or sexual cells.
It has been observed that if gametes escaping from different
filaments meet, they coalesce in pairs (Fig. 6, E, F, G).
They show that type of syngamy called conjugation, in
which the two sexual cells are alike in form and structure,
though of distinct origin. There is here no differentiation
of sex. It is impossible to distinguish one as male and
the other as female. The result of the fusion is called
the zygote. It soon retracts its cilia, settles and grows,
and after a period of rest germinates, dviding its contents,
which then escape as zoo-spores.

Ulothrix is an interesting instance of rudimentary
sexuality. It shows syngamy ; but the gametes are
isogametes. In form they are like the zoo-spores, except
in the number of the cilia. But after fusion the zygote
has like them four cilia. There are differences in size
and behaviour of the zoo-spores, some resulting from
repeated divisions of the parent cell, being smaller than
the type, though still having four cilia ; and they germi-
nate like them. Further, it has been seen that sometimes
the gametes themselves may germinate without fusing.
These facts have an important bearing on the theory of
origin of the gametes in so simple a plant. They suggest
that the gamete is really a zoo-spore reduced in size and
quality as a consequence of repeated divisions of the
parent cell. And that fusion, or syngamy, between such
gametes of a distinct source gives an impetus to new
development in these weakened cells. When we further
reflect that the motile is probably the primitive state of
these cells, and compare them with such an organism as
Euglena, it seems probable that sex may have arisen as an
offset against a weakening of the cells by divisions repeated
more rapidly than their substance is increased by growrth.

THE ORIGIN OF SEX IN PLANTS 15

The immediate effect of such simple syngamy as this
is a reduction in number of cells from two to one. A
consequence of it is an opportunity for the pooling of
the heritable qualities of two cells, which have been of
slightly different origin. These are the fundamental
lessons taught by Ulothrix, a plant in which the sexes
are not distinguishable from one another.

Whether in Animals or Plants, it is only in the simplest
cases that the distinction between the two sexes is absent.
There is evidence from both sources that a difference
between the sexes has been acquired by gradual steps.
In no group of Plants is this more clearly shown than
in the Brown Seaweeds, the best known of which is the
Common Bladder Wrack (Fucus vesiculosus), present on
all shores. This plant, however, with its broad leathery
fronds, is itself an advanced type of its class. Some of
the Brown Seaweeds are simple and filamentous ; and
it is among these that the most primitive conditions of
sex are found. In fact, complexity of structure and
elaboration of sex run parallel in them. For instance,
in Ectocarpus siliculosits, which consists of delicate
filaments, partitioned sporangia are found at the ends of
the branches, which open when ripe, and allow their
contents to escape into the sea-water as motile gametes
(Fig. 7). In form and size these are all alike, being
pear-shaped, with two cilia attached laterally ; while
within the protoplasm is a nucleus, and a red eye-spot.
But though the sexual cells are thus isogametes as regards
form, those from distinct sporangia differ in their behaviour,
so that they may be ranked functionally either as male
or female. For it is found that the gametes from certain
sporangia are at first motile, but soon lose their motility,
and attach themselves to some solid substratum. These

i6

SEX AND HEREDITY

may be regarded as functionally female, and they act as
centres of attraction to gametes from other sporangia,
which gather round them in crowds, coming into contact
with them by their advanced cilia (Fig. 7 (i)). These
gametes which retain their motility may be held to be

\

\

FIG. 7.

EC t near pus siliculosns. i. female gamete surrounded by a number of male
gametes, seen from the side ; 2-5, stages in the fusion of gametes; 6, zygote
after 24 hours ; 7-9, fusion of the nuclei in conjugation, as seen after fixing and
staining. (1-5 after Berthold ; 6-9 after Oltmanns. FromStrasburger.)

male. Finally, the cilium of one of them gradually
shortens and grows thicker, the male gamete approaching
the female till fusion of the two is fully carried out (Fig.
7 (2, 3, 4, 5) ). The nuclei, which at first remained separate
(Fig. 7 (7, 8) ), also fuse together (9), and the zygote is
then uni-nucleate. After the fusion begins all the other
gametes move away, the attraction having ceased. The

THE ORIGIN OF SEX IN PLANTS

zygote then rounds off, forms a cell-wall, and presently
may germinate into a new plant. Here then is a primitive
distinction of sex, which is, however, only functional.
Structurally the sexual cells are isogametes ; but function-
ally the female may be recognised by its early loss of
motility, and by the attraction which it exercises over
the motile males.

v///

FIG. 8.

Ectocarpns secundus. i = nlament bearing male i £), and female ($) gametangia ;
ii = male gametes ; iii = female gametes ; iv-vi=stages of fertilization ; vii-viii =
zygotes. (After Sanvageau.)

The next step is a distinction in size between the two
types of gamete, and it is seen in another species of the
genus Ectocarpus, viz. E. secundus. In this plant the
partitioned sporangia are of two different kinds, which
are borne on the same individual. One is small-celled,
and gives rise to small male gametes (Fig. 8, i, $, ii) ; the
other is larger-celled, and gives rise to larger female
gametes (Fig. 8, i, ?, iii). Both kinds of gametes are,
however, motile, and have the form usual in the Brown

S.H.

iS SEX AND HEREDITY

Seaweeds : being pear-shaped, with two cilia attached
laterally. The female gamete acts attractively upon the
smaller actively motile males, which collect round it
(Fig. 8, iv, v). Finally one fuses with it ; it then retracts
its cilia, settles, and germinates. Here then is a step
in advance of E. siliculosus. The sexual distinction is
not merely functional, but is marked by difference of
size, and that difference can already be seen in the spor-
angia that give rise to the gametes (gametangia) .

A further step in the distinction of sex appears in
Cutleria, an Alga with a narrow thong-like frond, which
bears gametangia of two distinct sorts. The one kind is
small-celled, and produces a small male gamete from each
cell. The other is larger-celled, and produces large female
gametes. Both, though differing greatly in size, have the
characteristic pear-like form with two cilia attached
laterally ; and at first both are motile. But soon the
larger female gametes retract their cilia, and lose their
motility, and round off to a sphere, with a clear receptive
spot. The male gamete which retains its motilit}^ is
attracted to it, and fuses with it as before (Fig. 9). The
points of advance here are the greater difference in size of
the gametes, and the loss of the motility and rounding off
of the female before fertilization. \Ye may now distinguish
the smaller male gamete as a spermatozoid, and the larger
female gamete as an ovum, or egg.

The next step is illustrated in Fucus, in the fact that the
large spherical female gamete, or egg, is never motile at all,
while the small spermatozoid retains its motility. The
large leathery frond of the Common Wrack is fertile at
the ends of some of its branches. Flask-shaped cavities
are there borne (conceptacles), and they contain the
sexual organs. In some species they are both borne

THE ORIGIN OF SEX IN PLANTS

together on the same plant (F. vesiculosus) , in others the
male may be borne on one individual plant and the
female organs on another (Fucus serratus). Thus there
may be in Fucus not only a distinction of sexual cells but
also a sex-difference in the plants that bear them.

FIG. 9.

Cutleria, showing the smaller male, and larger female gametangia. Top-left are
small male gametes ; top-right are large female gametes. Below are three stages
of fertilization. (After Reinke.)

The sexual cells of Fucus differ greatly in size. The
eggs are. produced by division of the protoplasm of a
single large cell into eight parts. When ripe these are
set free into the sea-water as dense, non-motile spheres,
large enough to be seen with the naked eye (Fig. 10 (5) ).
Each is a naked mass of granular protoplasm, with a single
nucleus at the centre. The antheridial cells are smaller ;
and each produces, by cleavage of its protoplasm, sixty-
four minute spermatozoids (Fig. 10 (1-4) ). But notwith-

20

SEX AND HEREDITY

standing the difference of size and cleavage, the anther-
idium and the oogonium hare features in common, which
point to their having originated in Descent from a common

FIG. 10.

Gametes and fertilization in Fncits. i, group of antheridia, borne on a
branched hair; 2, part of an antheridium showing development of sperma-
tozoids; 3, spermatozoid : a=eye-spot, £=nucleus; 4, isolated antheridia
liberating the spermatozoids ; 5, egg surrounded by spermatozoids ; 6, section
through a fertilized egg; sA = nucleus of egg, spk = sperm-nucleus, sp = sperma-
tozoids. (i, 4, 5, after Thuret ; 2, 3, after Guignard ; 6, after Farmer. From
Strasburger's Textbook.)

source. The spermatozoids are of the form usual for the
gametes of the Brown Seaweeds (Fig. 10 (3) ). If sea-
water containing active spermatozoids be added to water
containing living ova, the latter attract the former, which,

THE ORIGIN OF SEX IN PLANTS 21

being motile, collect around them in large numbers, coming
into close contact with their surface (Fig. 10 (5)). One
of them penetrates into the egg, and its nucleus has been
followed in its course inwards to the central nucleus of
the egg (Fig. 10 (6) ). Finally the two nuclei fuse together.
These details have been successfully observed by Professor
Farmer, who notes how immediately after the entrance
of the fertilizing spermatozoid into the ovum all the rest
leave it, as though repelled by some common impulse.
The fertilized egg, or zygote, may then grow directly into
a new Fucus plant.

This series of Brown Seaweeds probably indicates the
general course which the differentiation of sex has taken
in primitive Plants. The strength of the argument that
it does so lies not only in the gradual steps which they
show, but also in the fact that these observations do not
stand alone. Other large groups illustrate the same
thing. For instance, those closely related Green Algae
which are called the Siphonales and Siphonocladiales,
afford a series of steps which are quite comparable with
those seen in the Brown Seaweeds. Starting with the
conjugation of equal gametes, as it is seen in Acetabularia
(Fig. n, i), we arrive by steps of increasing inequality in
size and behaviour at the complete distinction of sex seen
in Vaucheria (Fig. n, v). Here, as in Fucus, there is a very
large immobile egg, which is fertilized by a very small
motile spermatozoid. Other groups show also a like
progression from isogametes to spermatozoid and ovum,
distinguished by size and behaviour. Such progressions
may be matched in Animals as well.

The question naturally arises why such progressions
should appear in several distinct evolutionary lines. That
the differentiation of sex has occurred more than once

22

SEX AND HEREDITY

makes it seem probable that some real advantage has
prompted it. The advantage appears to lie in the fact
that the larger the amount of food that is contained
in the egg the better nourished the offspring will be at
its first stages, and the better accordingly will be the
chance of its passing successfully through the dangerous

FIG. ii.

Examples showing increasing difference in proportion of the pairing
gametes in the green Siphonocladiales, and Siphonales. (After
Oltmanns.) i, isogametes of Acetabularia; ii, unequal gametes of
Bryopsis ; iii, unequal gametes of Codiitm ; iv, motile spermatozoids and
non-motile egg of Sphaeroplea ; v, large non-motile egg, and minute
spermatozoids of Vaucheria.

risks of youth. But the larger the egg the less mobile
it will be. Even in a fluid medium a large body is less
easily moved than a small one. We naturally associate
this with the fact that the larger eggs have lost their
motility. Motility of the egg is, however, immaterial so
long as the spermatozoids remain small and actively motile,
provided the egg can influence their movements so that it
shall act as a centre of attraction ; and this we have seen

THE ORIGIN OF SEX IN PLANTS 23

to be the case. Such advantages as follow from the
pooling of the hereditary factors of the two sexual cells
can still be secured by such means, notwithstanding the
loss of motility of the enlarged female gamete. Thus
the nett advantage lies with the plant : for without
sacrificing the benefits that follow from syngamy it can
still secure for its offspring the probability of successful
germination. Conjugating organisms, with their equiva-
lent gametes which are usually small, may be regarded
as a plant-proletariat that produces numerous offspring
with little physiological capital ; so that each individual
when produced must depend chiefly on its own efforts.
The organism which shows differentiation of its gametes,
with an enlarged, well-nourished egg, is like a capitalist,
whose progeny starts life well furnished with an inheritance.
To them the initial struggle for life is less intense. Other
things being equal, ultimate success should lie with the
latter : and a study of the vegetable Kingdom from this
point of view shows how successful the results of the
differentiation have been.

All of the higher forms of Vegetation have the sexes
fully differentiated. They have progressed on the footing
•of the relatively large, immobile, well-nourished egg.
In many of them the comparatively small spermatozoid
is still motile ; but in the Higher Flowering Plants even
this motility is lost, in accordance with circumstances
which will be explained in the second Lecture. At the
moment no more can be done here than to state the
leading facts of sexuality as seen in the Land Vegetation,
which stands higher in the scale of Evolution than the
water-plants hitherto discussed. Two further examples of
sexual propagation must suffice for the present, viz. a
Fern, as illustrating the lower types of Land- Vegetation ;

SEX AND HEREDITY

and a Flowering Plant, as exemplifying the highest point
reached in the Evolution of Plants.

". - J

Antheridia and spermatozoids of a Fern (Nephrodium). 4, 5, Mature antheridia
containing spermatocytes ; 6, rupture of an antheridium in water, and escape
of the spermatozoids"; 8, a single spermatozoid more highly magnified. (After
Kny.)

A Fern is a large leafy plant, and familiar examples
are the "Male" Fern and the "Lady" Fern. But
these are quite erroneous names, for the leafy plant is

THE ORIGIN OF SEX IN PLANTS

neither male nor female. It is neuter, bearing no
organs of sex. The spores, which are commonly borne
on the backs of the leaves in little brown capsules,
when they are shed upon the ground, germinate, and
produce each a small green scaly structure called a
prothallus. This is the sexual generation, and it bears
the organs of sex. The sexual cells, or gametes, pro-
duced by these differ widely in size and behaviour. The

a

FIG. 13.

Archegonia of Fern (Polypodiuni). A, is still closed; 0 = the ovum or egg;
B, shows an Archegonium with the canal of the neck open, and ready for fertiliza-
tion. ( x 240. After Strasburger.)

male gametes, or spermatozoids, are produced in large
numbers within the hemispherical aniheridia (Fig. 12 (1-5) ),
from which they escape on rupture caused by swelling
in presence of water (Fig. 12 (6) ). Each is a spirally
coiled body, and shows active screw-like movements in
the water into which it escapes. The movements are due
to the lashing action of numerous fine cilia (Fig. 12 (8) ).
The form of this male gamete is different from those
in the Algae previously described. But in its small
size and active motility it resembles them ; and in
both cases the gamete is a naked living cell, including
a nucleus. The female gamete or egg of a Fern is also

26

SEX AND HEREDITY

a nucleated primordial cell ; but it is much larger than
the spermatozoid, and is not motile. It lies protected in
a flask-like sheath called an archegonium, while at maturity
the neck of the flask is open (Fig. 13, B). For the

B

A

FIG. 14.

Fertilization in a Fern (Onoclea). A=a vertical section through an open arche-
gonium, probably within ten minutes after the entrance of the first spermatozoid.
( x 500.) B — a vertical section of the venter of an archegonium containing sperma-
tozoids, and-the collapsed egg with a spermatozoid within its nucleus. Thirty
minutes. ( x 1200. After Shaw.)

archegonium also ruptures by swelling with water, and
a channel of access to the egg is thus provided. The
spermatozoids enter that channel in numbers (Fig. 14, A),
and one of them finally penetrates the egg (Fig. 14, B) . Its
nucleus, still preserving the spiral form, may even be found
embedded in the nucleus of the egg (Fig. 14 bis). But
finally the two nuclei coalesce completely. The two

THE ORIGIN OF SEX IN PLANTS j;

unequal gametes fuse intimately, and the resulting zygote
is the starting point for a new Fern. Thus syngamy in
a Fern consists in the fusion of two cells differing in
character, and derived from distinct sources, to form a
zygote which grows into a new individual. The one is
a large non-motile egg, the other is a small motile sperma-
tozoid. In its essential features this corresponds to what

FIG. 14 bis.

Section of an egg of a Fern, showing the spirally coiled male nucleus
within the female nucleus, and fusing with it. Twelve hours after
fertilization. ( x 1200. After Shaw.)

is seen in simpler plants, though the details and accessories
are different.

In Flowering Plants the accessory circumstances are
again different, though the essentials are the same. The
parts which produce the sexual cells are grouped in that
complex structure known as the Flower. The most showy
parts of the flower, the petals, take no direct part in
reproduction. It is the parts that lie within the petals,
viz. the stamens and carpels, which produce the gametes

28

SEX AND HEREDITY

(Fig. 15). The stamens produce pollen-grains. But these
are not themselves gametes ; the grains after transfer to
the leceptive part of the carpel, which is called the stigma,
germinate, and each forms a pollen-tube. Within this
two cells are produced which are the male gametes them-
selves (Fig. 16, A). The female gametes are formed

FIG. 15.

Flower of the Quince, in median section, showing the sepals (sep), petals (pet),
stamens (st), and carpels (c). The ovary (ov.) contains the ovules. (After Church.)

within the carpels, which occupy the centre of the flower.
Each carpel encloses one or more ovules which develop
into seeds. The young peas within a pea-pod are familiar
examples of such ovules covered in by the carpellary leaf.
Deeply seated within each ovule is a single egg. This is
the female gamete, which is to be fertilized by one of the
male gametes (Fig. 16, B). At the moment we need not
consider the mechanism by which the junction of these

THE ORIGIN OF SEX IN PLANTS

29

gametes is brought about. This will be described in the
next Lecture. The immediate point is that the relatively
small, and in this case non-motile male gamete is conveyed
to the relatively larger ovum, both being primordial cells
without cell-wall. The two gametes coalesce. At first
their nuclei can still be distinguished, but gradually they
become fully fused together (Fig. 17). The resulting

B

A, Pollen- tube of Orchis with the male gametes (g) within; B, Pollen- tube
of Orchis entering the " micropyle " of the ovule, so as to convey the male gametes
to the ovum, which is the large cell more darkly shaded.

zygote gives rise to the embryo, which grows into the new
individual. Here again, syngamy consists in the coales-
cence of two cells, differing in character, and produced
from distinct sources, to form a new cell ; though again
the contributory circumstances are different. Such
examples illustrate what is the general fact for all the
Higher Plants, that the differentiation of sex established
in the lower forms is maintained throughout the higher

30 SEX AND HEREDITY

terms of the Vegetable Kingdom. The female cell, or
egg, is relatively large and non-motile ; the male cell is
relatively small, and in more primitive forms it is motile
in water, but in more advanced Plants of the Land that
motility is finally lost.

It thus appears that though very primitive organisms
may be sexless, there is a fundamental unity of the
method of sexual propagation in Plants, when once it

Fie. 17.

Fusion of male and female gametes of Lily. (After Mottier.) A, shows the
vermiform male nucleus applied to the egg-nucleus (Liliitin Martagon) ; B, shows
the egg-cell of Lilium candiduin with the two sexual nuclei fusing. The nuclear
membranes have disappeared at the place of contact.

had become fully established. In certain of the rudi-
mentary examples of sex the gametes may be indistinguish-
able as male or female. From such simple beginnings
we have been able to trace the steps of differentiation of
sex. Indications of a gradual increase in size of the
female gamete, or ovum, and of its loss of motility have
been seen ; while the male spermatozoid remains relatively
minute and active. A reasonable biological explanation
of this has also been offered. A full sexual differentiation
of this nature was already attained in the more advanced

THE ORIGIN OF SEX IN PLANTS 31

Algae. The behaviour, and often approximately the
proportions of the pairing gametes of the lower vegeta-
tion of the Land, such as the Mosses and Ferns, remain
substantially the same as in the Algae. From Fern-like
plants a gradual transition has led to the state seen in
the Flowering Plants. But though in them the male
gamete is no longer motile, the fusion of the gametes
is still a coalescence in which the nuclear fusion is an
essential feature. When stripped of all accessories, many
of which find their explanation in the varied circum-
stances under which plants live and propagate, the
actual fact of sexuality has remained the same for them
all. We conclude then that syngamy consists for Plants
at large in the coalescence of two sexual cells of more or
less distinct origin, and especially of the nuclei which those
cells contain. There may be, and there are, differences in
the mechanism by which this syngamy is brought about
in Plants of various habitat and character. The next
Lecture will be devoted to a study of those differences
of method, and to the varied circumstances to which
those differences may be ascribed.

LECTURE II

THE EFFECT OF A FIXED POSITION ON THE
SEXUALITY OF PLANTS

IF an average man were asked what is the most striking
difference between Animals and Plants, he would probably
reply that Animals move and Plants do not. But this
would be an over-statement of the real facts. Living
Plants do move, though their movements are slow and
constrained. No organic Life is possible without move-
ment of one sort or another. But there is an essential
difference of structure between Animals and Plants which
explains their respective powers of movement. The
protoplasm of the former is not as a rule confined within
a wall, and tissues-masses composed of such cells can
move freely, as our own muscles do. But each of the cells
of the Plant is enclosed in a resistant cell-wall, which
checks the mobile protoplasm within, and at the best its
movements are only slow. Plants have in fact bartered
their free motility for the protection given by the cell-
wall. Already Euglena shows in its temporary encysted
stage the condition usual in the plant-body (Fig. 3, D) ;
but the cell-wall is a permanent feature in such simple
plants as Protococcus viridis (Fig. 5) ; and in Ulothrix
(Fig. 6, A) it is also, except in its propagative phase.
The further circumstance that Plants are habitually fixed

SEXUALITY OF PLANTS 33

to the substratum, as are Ulothrix or Fucus, or even
rooted in the soil like Ferns and Flowering Plants, effectu-
ally prevents their movements as a whole from place to
place. In respect of sexuality this imposes a vital
difference. The mobile Animal is free to seek its mate ;
the encysted and rooted Plant is not. Hence the whole
problem of sexuality for the Higher Plants appears to
be a different one from that of the Higher Animals.
Nevertheless in both a very similar coalescence of gametes
is the end to be attained, and there are various analogies
in the means employed to attain that end.

In Plants that live in the water the fact that all except
the simplest are non-motile as a whole, and fixed to the
substratum as Seaweeds are, does not present any serious
obstacle to success. For they are mostly gregarious, and
one of the gametes or both are commonly motile in the
water into which they escape. In those which are
sexually differentiated the male commonly retains, as a
spermatozoid, its power of movement from place to place :
and Algae so provided are believed to represent the remote
ancestry of the Land- Vegetation. There is no need in
such cases for both gametes to be motile, if the gamete
which is sedentary can control the movements of that
which is motile. That it can do so is demonstrated
by any mixtuie of water containing living spermatozoids
of FUCKS with water containing its ova (Fig. 10 (5) ). The
influence of attraction before syngamy and of repulsion
of the remaining spermatozoids after syngamy suggests
that the power of the ovum lies in diffusion from it into
water of some soluble substance, attractive or the reverse.
A study of the cognate phenomenon in Ferns has shown
that this is a true explanation. But it would only serve
for organisms in which a water-medium is available at

S.H.

34 SEX AND HEREDITY

the. time of fertilization. This immediately raises the
question of how syngamy is effected in the Ferns and
other primitive Land-Living Plants.

The method of syngamy in Ferns may be held to
represent fairly that of all the more primitive Plants of
the Land. Its main features have been described in the
previous Lecture. The spermatozoid, set free and motile
in water (Fig. 12 (6, 8)), and the ovum, deeply seated in
the archegonium (Fig. 13), are the gametes. The problem
is to bring them together with certainty. The medium
of transit is water. It is only in presence of water that
the antheridia and archegonia open. In Nature this is
provided by showers, or copious dews, and into that
water the spermatozoids escape. A significant fact is
that the spermatozoids are very numerous. But still
the prospect of the fusion of spermatozoid and ovum
being carried out would be almost infinitely small were
it left to mere chance. The ovum, lying protected in
the cavity of the archegonium, would almost inevitably
be missed in random wanderings of even numerous
spermatozoids. But any microscopic preparation of
them in the living state shows that the spermatozoids
are attracted, and enter the archegonium with certainty,
and in large numbers. Experiment has explained the
source of the attraction.

If artificial archegonia be made in the form of minute
glass flasks, it would be possible to fill them with solutions
of various soluble substances. If they were then immersed
in water the soluble substance would diffuse out, the neck
of the flask being constantly the centre of greatest con-
centration. If the water contained living spermatozoids,
the effect of each substance used could be noted, according
as it influenced their movements. In this way a number

SEXUALITY OF PLANTS 35

of substances have been tried, and of various strengths.
It has been found that a solution of malic acid, of strength
about o-ooi p.c., diffusing out into water, serves as a
positive attraction, leading the spermatozoids to the
neck of the flask, which they actually enter as they
would a real archegonium. It is therefore concluded as
probable that a soluble substance, similar in its action
to malic acid, is given out from the ovum, and serves
to direct the movements of the spermatozoids.

The deeply seated position of the ovum in a Fern is
clearly an advantage in the protection and nutrition of
the fertilized egg (Fig. 131. The maternal tissue closely
surrounds the embryo at first. A connection is kept up by
means of a suctorial "foot," between the embryo that grows
from the egg and the parent prothallus (Fig. 18) . This per-
sists until the young plant is established so as to be able
to nourish itself by its own root and leaf. On the other
hand w'e see that the position of the ovum at the base
of the flask-shaped archegonium offers no serious obstacle
to syngamy, provided the attraction of the motile sperma-
tozoid is as effective as experiment proves it to be. This
has been the method of sexual propagation of all the
primitive Plants of the Land. They are represented by
the Mosses, Ferns, Horsetails, and Club-Mosses. Such
Plants proclaim their aquatic origin by retaining the
ancestral method of syngamy through water. They are
not typical Plants of the Land, but might be properly
called the Amphibians of the Vegetable Kingdom. They
have, speaking figuratively, one foot on land and one
still in the water. They cannot complete the cycle of
their life in its most critical point, that of the sexual
production of a new individual, except when external
fluid water is present. Without it the spermatozoids

SEX AND HEREDITY

are not liberated, nor do the archegonia open. It is
possible by watering cultures only by absorption from
below to grow the sexual plants of Ferns or Mosses for
long periods without any sexual propagation at all.
This is a restricted existence from which the Higher
Land-living Plants have finally broken away. In the

FIG. 18.

Kmtryo of a Fern (Adia;itiiin\ embedded in the tissues of the parent prothaUus.
where it is protected and fed by the surrounding tissues. L = leaf; .K = ro >t :
S = stem; F = suctoria! foot. (After Atkinson.)

Seed-Plants, as we shall see, external fluid water is no
longer necessary for syngamy.

In the previous Lecture it has been seen that the bare
facts of syngamy in Flowering Plants correspond to those
in lower forms. The male gamete is produced from
the pollen-grain. The female gamete, or egg, lies deeply-
seated in the ovule. These gametes fuse together to
form a zygote that develops into the embryo. But as

SEXUALITY OF PLANTS 37

all ordinary Seed-Plants live on land, and neither the
one gamete nor the other is set free into water, nor is
motile, it is clear that the mechanism that brings about
the fusion of gametes in Flowering Plants must be
different from that in the Algae, or in the Ferns. It
involves two stages. First the transfer of the pollen-
grain from the stamen where it is produced to the recep-
tive surface of the stigma : this is called Pollination.
The second is the transfer of the male gamete, derived
from the pollen-grain, to the ovum with which it fuses :
this fusion is called Fertilization, or Syngamy. The two
stages are quite distinct in their nature, and should be
studied separately. Pollination is only a means to the
end : Fertilization is the end itself.

In Pollination the distance through which the pollen-
grain must travel from the stamen to the receptive stigma
varies greatly, and depends upon the structure of the
flower in question. ^ Some flowers, which are called
hermaphrodite, contain both stamens and carpels ; in
that case the distance to be traversed may be small
(Fig. 15). But in many plants the stamens and carpels
may be borne on different flowers, as in the Hazel,
Beech, or Oak ; or even on different plants, as in the
Campion or Willow. There are thus various degrees
of separation of the sexes in the Space which has
to be traversed by the pollen-grain. But a separation
in Time of maturity is equally a cause of difficulty in
pollination, and it may apply even in hermaphrodite
flowers. For if the pollen is matured either before or
after the stigma of the same flower is ready to receive
it, clearly to be effective the pollen must be brought from
a distance. Thus pollination is not so simple a problem
as it looks at first sight.

38 SEX AND HEREDITY

There is also another point to be considered. Within
certain limits a difference of origin of the fusing gametes
is an advantage. Already in Ulothrix and in Ectocarpus
the conjugating gametes have been seen to arise from
different sporangia. In Flowering Plants it has been
shown in many cases that intercrossing gives on the
average a larger and stronger progeny. By crosssing
is meant that the pollen which produces the fertilizing
gamete shall have been derived from a flower or plant
distinct from that bearing the ovum which is to be ferti-
lized. Seeds that result from such a crossing have been
found to be on the average more numerous and heavier
than those resulting from self-fertilization. The course
of Evolution of Flowers has been such as to secure this
advantage. The effect of the separation of stamens and
carpels in space, and in time of maturing, is to promote
intercrossing. But all such developments have still
further complicated the mechanical problem of Polli-
nation for Seed-Plants.

vSuch Plants being themselves immobile, as naturally
follows from their being rooted in the soil, use is made of
outside agencies, such as the movements of \Yind and
Water, or the mobility of Animals. The mechanism of
flowers has been specialised in the most remarkable
manner in accordance with these methods of transfer.
Where use is made of Wind, as in the Grasses, the
flowers produce abundance of dry dusty pollen, easily
shaken out in clouds from anthers balanced on very
flexible filaments. The stigmas meanwhile are much
branched and feathery, so as to expose a large surface for
catching the grains. These features go with close
grouping of the flowers, which are individually small and
inconspicuous (Fig. 19). Where animals are the active

SEXUALITY OF PLANTS

39

agents, the flowers are attractive and conspicuous by
their scent, by honey-secretion, and by widely expanded
floral envelopes of bright colour. The latter attract the
eye, the former the other senses of the animal, and lead
him to visit the flower for his own purposes of gathering

FIG. KJ.

A spikelet of a Grass, showing one flower in bloom, with three anthers on their
flexible filaments, and two feathery stigmas. These, with the inconspicuous size
and colour, are common characters of plants pollinated by agency of wind.

honey, or pollen. Incidentally the floral mechanism is so
arranged, in size and form of its parts, that as he visits
the flower, pollen, often of a sticky nature, is deposited
on his body. The flower may be so formed as to lead
him to take a definite position, so that the pollen is
deposited on a definite part of his body. The result of
a succession of visits to a succession of flowers of like
construction will then be that, if the stigmas correspond

SEX AND HEREDITY

in position to the spots where he bears the pollen, some
may be deposited upon them. Thus unwittingly he will
have been the agent of transfer of the pollen from the
pollen-sac to the receptive stigma (Fig. 20).

Such mechanisms have been elaborated in the course of
Descent in an infinite variety of detail. This is the
biological meaning of the attractive features of form,
colour, and scent which flowers have assumed. It may

FIG. 20.

Pollination of Salria. i = flower visited by Humble Bee, showing the projection
of the curved connective of the anther from the helmet-shaped upper lip of the
corolla, and the deposition of the pollen on the back of the Bee; 2=an older
flower, showing the elongated style with its stigma in such a position as to receive
pollen brought by a Bee from a younger flower ; 3-4, show the mechanism by
which the anther swings when pressed forward (as shown by arrow) by the proboscis
of the Bee. (After Strasburger.)

even be seen how certain floral types have been adjusted
in relation to the visits of certain animals, and show
development parallel with them. A good instance is that
of the Aconite and the Humble Bee, in which the size
and shape of the flower is such as to accommodate the
animal. A study of their distribution across Europe and
Asia shows that the northern limit of both almost exactly

•j

coincides. This suggests the importance of the Humble
Bee in the transfer of the pollen of the Aconite, while
the food which the flower offers may in some measure

SEXUALITY OF PLANTS 41

react in determining the distribution of the Bee. The
methods of transfer of the pollen may be very varied.
But the essential feature of them all is the same, viz.
the conveyance of an immobile body essential to pro-
pagation from the pollen-sac where it is produced to the
surface of the stigma, where it can germinate.

The study of the structure of Flowers as pollinating
mechanisms has caught public attention, and the facts
are often presented in sensational language. Floral
mechanisms and their evolution parallel to the forms of
their visitants are certainly wonderful instances of adapta-
tion. But in studying them it should always be re-
membered that it is the immobility of the Plant that
gives these adaptations their special value. Vegetation
was originally aquatic. The spread of Plant Life to the
Land raised a thousand difficult life-problems. One of
the most urgent was how it was possible for Plants, being
immobile, to maintain sexual reproduction under con-
ditions of life in air instead of in water. A first step in
the solution of that problem, the transfer of the immobile
pollen, is carried out in that wonderful structure, the
Flower, which is as beautiful to the understanding as it
is to any of the senses.

Once landed on the surface of the stigma the pollen-
grain germinates and forms a pollen-tube, wrhich, pene-
trating the tissue, traverses the carpel downwards to the
cavity in which the ovule or ovules lie (Fig. 21). There it
is led to the apex of the ovule ; it enters the micropyle,
and impinges directly upon the embryo-sac where the egg
is attached. Since it conveys two male gametes, these
can be discharged at the apex into the embryo-sac. It
is one of these which entering the egg fertilizes it. These
are, briefly told, the steps leading to fertilization or

SEX AND HEREDITY

syngamy in Flowering Plants. They will now be con-
sidered in detail.

The germination of the pollen-grain takes place normally

on the stigma, and the course of
the pollen-tube can be followed,
as in Fig. 21. But germination
can also be induced in a nutri-
tive medium apart from the
stigma, such as a solution of
cane sugar of suitable strength.
This makes it possible to ob-
serve the origin and behaviour
of the pollen- tube. The ger-
mination may be very rapid.
From fresh Pollen of the wild
Hyacinth placed in 7-10 p.c.
solution pollen-tubes will be
formed in about fifteen minutes,
and in an hour will have grown
to a length several times the
diameter of the grain. The
effect of external influences
upon the growth of the tube
FlG- 2I- can be studied in such cultures.

Ovary ot Polygonum during fertiliza-
tion, containing one straight ovule. fs= por instance, if grains be
base of ovary; //< = fuuiculus ; cha =

chalaza; »» = nucellus; ;»*' = micropyle ; Urinated Ulldcr a CO
tt = inner, and ;> = outer integuments;

«= embryo sac; «ft=cential nucleus {he tubes filSt isSUC
of sac; ei = egg-apparatus ; an = antipodal

ceiis; g=styie; n= stigma; />=poik-n- indiscriminately in all direc-

grains ; />s=pollen-tubes. ( x 48. After

su-asburgcr.) tions. But soon those near the

margin turn inwards from the free air, that is, they grow
away from the source of oxygen (Fig. 22). If a similar
culture be prepared, and a piece of the style and stigma
of the same species be introduced, the tubes curve towards

SEXUALITY OF PLANTS

43

V

o

Pollen-grains germinated in a nutritive
medium, under a cover glass, of which
the margin is shown. The tubes curve
away from the margin, that is, away from
the supply of oxygen. (After Molisch.)

FIG. 23.

Result of culture of pollen tubes of
Xarcissns Tazetta, in the neighbour-
hood of the style and stigma, in 7 p.c.
sugar, after sixteen hours ; diagram-
matic. ( 10. After Molisch.)

it, and especially towards the cut surface (Fig. 23). They
also tend when grown exposed to the air to follow a
moist surface. These three fac-
tors all influence the growth of
the tube in the same way, when
pollen germinates on the stigma.
Turning away from free air, ad-
hering to the moist surface, and
attracted by the tissue of the
style, the tube penetrates it.
Sometimes there is a channel
down the style filled with mucil-
age ; in \vhich case the tube
grows down it, and does not
penetrate the tissue. In other FIG. 24.

,1 11 c i • Pollen-grains of the Corn Cockle

CaSeS the CellS OI tlie Stigma (Agrostemma) germinating on the

r 11 . -, stigma, and the pollen-tubes pone-

are themselves perforated by the trating its tissue. ( After Strasbuvger.)

44

SEX AND HEREDITY

tube, which then passes on between the cells of the style
(Fig. 24). Traversing the style in this way, it reaches
the cavity of the ovary, where it may be conducted

FIG. ->5.

Median section of an ovule of the Marsh Marigold (Caltha), showing the egg-
apparatus (f.rt) deeply seated, and consisting of the single ovum, which projects into
the embryo-sac, and two adjoining cells (synergidae). /=fuiiieulus ; r/; = chalaza ;
;» — mi( -ropyle ; HUC-- - nucellus ; o. int.; i. int. = outer and inner integuments ;
/;j = central fusion nucleus ; unf antipodals. The structure here shown is very
general for ovules of Flowering Plants. ( x no.)

mechanically by directing hairs to the apex of the ovule.
It there enters the narrow channel of the micropyle, and
makes its way to the ovum itself (Fig. 27, B). It may be
a question what are the influences which direct che last

SEXUALITY OF PLANTS

45

part of the course, but it is accomplished with a high
degree of certainty.

The tube is a means of conveyance of
the male gametes to the ovule. The
mature pollen-grain contains two cells
(Fig. 26). One, the larger, is a vege-
tative cell, and takes no direct part
in propagation. Shortly after germi-
nation begins the other divides into
two, which are the male gametes. On
germination these contents of the pollen-
grain pass into the growingtube, and ad- strasburser-
vance with it asit grows (Fig. 27, A ) . They are thus conveyed,
always protected within the tissue of the style, to the ovary,

FIG. 26.

Pollen-grain, showing two
cells : the larger is vegeta-
tive; the smaller, to the
right, gives rise to the two

540.

After

0

B ¥10.27. A

A, Pollen-tube of Orchis latifolia, teased out from the ovary. r = vegetative
nucleus; gg=gametes. ( x 500.) B, Pollen-tube of the same penetrating the
micropyle of the ovule: just below its lip is the egg-apparatus, with the ovum
shaded. The male gametes are still in the tube. ( x 300.) After Strasburger.

and finally to the ovule itself (Fig. 27, B). There they aiv
extruded through the soft tip of the tube into the embryo-

46

SEX AND HEREDITY

sac, and one of the two gametes entering the ovum, its
nucleus may be seen to fuse with that of the ovum (Fig.
28). The result of fusion of the non-motile male gamete
with the non-motile ovum is the zygote, and from this
the embryo of a new plant develops. Here then is a
syngamy in which neither gamete is capable of independent
movement so as to secure the fusion. The opportunity
for movement in water, after the ancestral fashion of the

FIG. 28.

Fusion of male and female gametes of Lily (after Mottier). A, shows the
vermiform male nucleus applied to the egg-nucleus (Lilinm Marl agon) ; B, shows
the egg-cell of Liliinn cam/niiini with the two sexual nuclei fusing. The nuclear
membranes have disappeared at the place of contact.

Ferns, is absent in accordance with the Land-Habit. The
fact that the flower is as a rule borne distally on the shoot,
and blooms habitually in bright sunny weather, precludes
the means of transit through water. In the course of
the Evolution of Seed Plants a new means of transit has
been substituted for it, better suited to their life on
Land. But the wonderfully adaptive growth of the
pollen-tube is in itself no more striking a phenomenon
than is the adaptive movement of the active sperma-
tozoid that it replaced. That it did replace it, and that

SEXUALITY OF PLANTS

47

Seed-Plants really were evolved from organisms which
were fertilized as Ferns are, is shown by facts recently
discovered. For certain primitive Seed-Plants have been
found to have motile spermatozoids (Fig. 29). But they
move only in the limited sphere of a small volume of fluid
within the ovule. These Plants have been driven by the
exigencies of their land-habit to secrete the medium in
which their spermatozoids move. & -

For Land-Plants such an archaic
method so artificially maintained is
clearly unpractical. The Cycads
and the Maiden-Hair Tree that
show it are rightly regarded as sur-
vivals, whose conservatism has
almost cost them their lives. The
rush of Evolution of Land-Plants,
with a more practical method of
fertilization, has passed them by.
But they at least survive to tell the

story of Descent, and to point its ( about m After Webber>
moral.

The transition from water to land has thus profoundly
affected the mechanism of sexuality in Plants, without
altering the essential features of the process. Syngamy
is still a fusion of two sexual cells of more or less distinct
origin to form one, which is the starting point for a new
individual. Witnessing the consequences of the change
of medium, the mind is impressed by a sense of the
vulnerability of primordial cells exposed to the air. In
water the risk is not great. The gametes may be voided
directly into water, as they are in Fuciis. Syngamy
is then carried out quite apart from the parent, and the
new individual is independent of parental nursing. But

FIG. 29.

End of Pollen-tube of Zainia,
showing the vegetative cell (v),
sterile cell (s), and two sperma-
tozoids. (a) before movement
has commenced ; (b) after the
beginning of ciliary motion.

t
from Strasburger.

48

SEX AND HEREDITY

in Land-Plants the egg is never shed. It is retained by
the parent. In the Mosses and Ferns it is embedded
in the flask-like archegonium, where it is still accessible
to the free-swimming spermatozoid through the open
channel of the neck. Thus mechanical protection of
the egg is ensured, as well as the nourishment of the new
germ by the parent after syngamy. In the Flowering
Plant this protection and nutrition of the germ is still

FIG. 30.

A, Transverse section of the pistil of Caltlia, showing the ovules protected in the
separate, pod-like carpels ; B, transverse section of the ovary of Lily, showing
the carpels united (syncarpous), closely enveloping the ovules. In both cases the
structure of the ovules themselves is substantially as in Fig. 25.

more effectively secured. It is noteworthy how deeply
seated are the ova, and protected by successive coats of
tissue. First they are covered in by the carpellary wall
(Fig. 30) ; then by the integuments of the ovule ; next follows
the tissue of the nucellus ; and lastly the primordial ovum
is contained in the embryo-sac, which lies centrally in
the ovule (Fig. 25). Such repeated sheaths give a very
perfect protection against exposure to the air, or to
mechanical damage. But they increase the difficulties
of fertilization. These are overcome by means of the
exactly directed growth of the pollen-tube. On their

SEXUALITY OF PLANTS

49

way to the egg the male gametes are never exposed in
land-living Plants. Even in the pollen-grain the cell
that gives rise to them is protected by its usually
yellow, corky wall from too
intense sunlight, and from
risks of evaporation from its
surface. After the first stage
is passed, the transfer of the
gametes by the pollen-tube
is consistently within the
protecting tissues of the
carpel. Doubtless these pre-
cautions are very necessary
in Plants growing exposed
upon the land, which bear
their flowrers containing the
gametes at the ends of their
branches. The protection
and nutrition afforded by -
the parent is thus so well
secured that a large num-
ber of eggs is unnecessary.
Only one is present in each
ovule. But this is handled

physiologically With extreme Seeds in median section. The upper is of

Shepherd's Purse, the lower of Datura. f=

Care. Each germ, Once estab- funiculus : »» =micropyle : t = seed-coat : e =

endosperm: c =cotyledous : £>/=plumule:

hshed by syngamy, is evi- »-=radicie. Enlarged.
dently a thing of individual worth (Fig. 31). Yet
Tennyson, the poet of the Darwinian era, has done scant
justice to the value set on each germ. Impressed
with the ruthless waste of Organic Nature he cries :

" How careful of the type she seems,
How careless of the single life/'

5o SEX AND HEREDITY

For once the Seer saw only a half truth. Had he
visualised with his accustomed clearness each germ, with
its deep-seated position, and its many protective coats ;
and realised how perfect are the conditions for nursing
the embryo in the maternal tissues, Literature might
have lost two epigrammatic lines, but Science would have
secured a truer word- picture. It is not at the outset,
but later in the individual life of Land Plants, that the
full weight of the physical struggle for existence comes.
In the period of embryology of new germs Nature appears
in her most engaging mood ; not " red in tooth and claw,"
but as the nursing mother. That role is none the less
attractive that it has been forced upon her by the ruth-
less conditions of Life on Land. In aquatic Plants, such
as Ulothrix, Ectocarpus, or Fucus, the zygote has at once
to meet unprotected the contingencies of Life. Many
fail, but many succeed in passing that less drastic ordeal
of early existence in water. But the ordeal of life in the
air is more severe to the young in many ways. Hence
all Land Plants, as a condition of the bare existence of
their germs, have adopted the nursing habit. The ordeal
of the young organism is thus deferred. The embryo,
nourished and strengthened through the nursing period,
is better fitted to meet it than the naked zygote would
have been. This is the biological aspect of the facts
of internal embryology. The parallel in this between
the Higher Plants and the Higher Animals is singularly
close. While noting it, we should always bear in mind
that the results in the two Kingdoms have been independ-
ently achieved. They may be held as evidence of separate
reaction to the exacting conditions of terrestrial life.
But the incidence of these conditions has been the same
for both the Kingdoms of Living Beings.

LECTURE III

THE REPRODUCTIVE PROCESS IN ANIMALS:
SOME OF THE GENERAL PRINCIPLES

PERHAPS the most wonderful thing in Nature is a living
animal. And there is nothing more terrible in its im-
pressiveness than to be a witness when such a living
creature is suddenly deprived of its life, and to see in
the place where it was a moment before a mass of inert
material bearing the same outward form that the living
creature had, but without that characteristic — life-
that gave it its all-transcending interest. What that
' life ' really is in its ultimate nature is an absolute
mystery, and there are many of us who believe that it
must necessarily remain forever a mystery. For the
main instrument upon which wre are dependent for its
investigation is our brain, and the whole activity of that
organ is merely a phase of that living activity the nature
of which it is our endeavour to understand.

Even when we leave on one side the ultimate nature
of Life itself and restrict ourselves to the study of its
more superficial phenomena we find ourselves up against
quite unexpected complexities. Take the case of a
human being and consider one of his very simplest
actions. What can be more simple than to stand still
doing nothing ? One can observe the phenomenon at

51

52 SEX AND HEREDITY

a street corner in any one of our great cities. If you ask
one of the individuals concerned who and what he is
he may reply that he is a worker — though the most careful
watching may fail to detect any signs of a desire to work.
What is the man really doing ? Standing up — what
more simple ?

But now suppose you had the body of that same man
frozen stiff — with every bit of him in precisely the same
position as it has been during life — and suppose you now
stood him up on his feet on the same bit of flat pavement.
You would find considerable difficulty in getting him
balanced so as to stand up at all, and if at last you did
succeed you would find that the slightest touch, or a slight
breeze, would overturn him. And you would learn the
lesson that there is something: essentiallv different between

{j J

the standing up of that mass of dead material and the
standing up of the living man. As a matter of fact, when
you look into the mere action of standing up you find
that it is a matter of fearsome complexity.

Suppose you had a chain of iron or wooden rods, jointed
loosely together end to end, you might be able — with a
good deal of trouble — to arrange it so as to be kept upright
by elastic bands passing from one rod to another and
each under exactly the proper degree of tension. Xow
this is the sort of principle which is at work in the living
human body — the rods are represented by the bones of
the skeleton and the elastic bands by the muscles which
pass from one bone to another and pull against one another
so as to keep the whole arrangement upright. But what
puts the human arrangement upon a totally different
level of complexity as compared with the rough model
I have described is that it is automatic and self-regulating.
The equilibrium of the body is constantly being interfered

REPRODUCTIVE PROCESS IN ANIMALS 53

with. A slight movement of the head or arm, a
slight puff of wind, a slight touch by a passer-by would
be enough to overturn the body wrere it not instantly
met by a slight automatic increase in the tension of
certain of the muscles. As a matter of fact, the body
when apparently standing still is never really standing
still for a moment : it is constantly commencing to fall
over to one side or the other, and is constantly being
automatically checked and pulled back towards the
vertical. Hundreds of bundles of muscle-fibres are at
work the whole time, all co-operating together, each
doing exactly its share — no shirking, no strikes, no lock-
outs, every unit loyally doing its bit. If our friend at
the corner were to realise that wonderfully co-ordinated
activity within his own body it would surely break his
heart : he would fling himself into the river ; he might
even take to work !

What I have been saying so far is merely to impress
upon you what unsuspected complexities underly even
the simplest actions of living creatures. The reproductive
process is not one of these simplest processes but probably
the most tremendously complex of them all. How it
is that a speck of matter so small as to be invisible except
under a powerful microscope, such that no details of its
intimate structure can be made out even with the very
highest magnification, can reproduce all the details of
structure and function of the human being, nay of an
individual human being with his obscure peculiarities
of appearance, of manner, or of habit, is a mystery which
must surely for all time transcend scientific knowledge.
And yet, though the ultimate nature of this as of other
vital processes is unknown, a vast and ever increasing
amount of knowledge has been accumulated regarding

54

SEX AND HEREDITY

the reproductive process in the Animal Kingdom, and in
these two lectures I propose to deal with some of the

general principles which have
been elucidated, leaving on
one side the phenomena of
Heredity, which will be ex-
pounded to you later by a
distinguished investigator of
this subject.

The essential features of
the reproductive process are
most easily grasped by study-
ing it in the lowest and simp-
lest animals forming the group
Protozoa — animals of minute
size, in which the individual
consists of a single cell which
creeps, or swims, or floats,
leading an independent exist-
ence. We will commence with
Copromonas, a creature which
occurs not uncommonly in
water in which Frogs are
kept. The creature consists
of a minute pear-shaped cell,
i.e. a mass of protoplasm con-
taining a nucleus (Fig. 32, N).
The outer layer of protoplasm

*

c if.

-AT

32.

Copromonas, as seen under a very high . , . -, ,-, ,rr j r

magnification (after Dobell). c.v, contrac- IS Slightly Stiffened, lOrmillg a
tile vacuole ; /, flagelluni ; f.v, food vacuole . -, . -, . 11-1 i • ^

thin SKin Or pellicle Which

(temporary stomach) ; N, nucleus ; r,
reservoir.

keeps the creature in its
definite shape. Projecting from the narrower end is
a long very thin thread of protoplasm — the flagelluni

REPRODUCTIVE PROCESS IN ANIMALS 55

(Fig. 32, /) — the end of which can be twirled round
in such a manner as to draw the creature forwards
through the water. The flagellum emerges from the
mouth of a slender slightly curved tubular ingrowth
of the pellicle, which serves as a gullet down which
tiny food-particles are swept by the current sent back
by the flagellum. Every now and then a droplet of
water laden with such food-particles may be seen to
detach itself from the inner end of the gullet, and travel
away through the protoplasm, forming a little temporary
stomach (Fig. 32, f.v), in which the process of digestion
takes place. The water which is taken into the body
in this process is eventually drawn out of the protoplasm
and ejected into a pocket-like reservoir (Fig. 32, r)
connected with the gullet, by a little pump known as the
contractile vacuole (Fig. 32, c.v), a bulb of protoplasm
which expands and contracts rhythmically.

The Copromonas under favourable conditions goes on
living, feeding and growing. Like other animals it does
not increase in size indefinitely, but after a time reaches
a rough limit of normal, as we might say, adult size.
Its increase beyond this is counteracted by what we
recognize as the simplest type of reproductive process-
the process known technically as Fission — in which the
individual simply divides itself into two new individuals,
each of half its size. The process is illustrated by the
upper portion of Fig. 33. The ordinary adult individual
(i) draws in its flagellum (2), and then begins to split
longitudinally, the process commencing at the front end
(3) and gradually extending until the individual is com-
pletely split into two new individuals exactly like itself,
except that they are of half its size. It will be noticed
that each of the young individuals has grown a new

56 SEX AND HEREDITY

flagellum, and that each is provided with a nucleus by

FIG. 33.

Diagram illustrating the life-history of Copromonas (after Dobell).

The upper circle of figures (1-5) illustrates the process of reproduction, the lower
(5-12) that of syngamy.

[The diagram includes two departures from the normal which are not alluded to
in the text, namely (a) a short-circuiting from 9-1 so as to omit encystment, and
(b) encystment without syngamy (i-n).]

the original nucleus growing out into a dumb-bell shape
and then becoming nipped across into two. Each of the

REPRODUCTIVE PROCESS IN ANIMALS 57

new individuals under favourable conditions behaves
just like its parent : it feeds, grows and eventually divides
again by fission. And so generation after generation-
each individual ends by resolving itself into two new
individuals.

But this process of fission does not go on indefinitely.
From time to time it is interrupted by an opposite kind
of process in which, in place of one individual becoming
divided into two, two individuals become fused into a
single one in the process known as Syngamy — illustrated
by the lower part of Fig. 33. Two individuals swimming
about come in contact by their front ends (6), adhere
together and become gradually merged into a single
individual shaped like the parent (7-10). An essential
feature of this process of syngamy is that the nuclei of
the tw^o individuals, after undergoing complicated changes
which need not be gone into, undergo complete fusion
together (10, u), just as the protoplasmic bodies do.

In the process just described we recognize a typical
case of syngamy : the t\vo individuals which fuse together
are gametes ; the single individual produced by their
fusion is a zygote. A very usual sequel to the process
of syngamy is well seen in Copromonas, in that the zygote
rounds itself off, surrounds itself with a protective shell
or cyst (n), and enters on a period of repose before it
emerges again and resumes its pear-shape and its active
swimming existence.

One of the most interesting things about Copromonas
is that while it shows typical Syngamy — the essential
phenomenon of Sex — there is no obvious sexual difference,
no obvious sign of maleness or femaleness. To study
these differences we will take two other minute Protozoa-
(i) Stylorhynckus, which lives as a parasite in the intestine

SEX AND HEREDITY

of a beetle named Blaps, occasionally found in cellars
and outhouses in our own neighbourhood, and (2) Plas-
modium, the parasite which causes one of the most
destructive of human diseases, namely Malaria.

In the case of Stylorhynchus (Fig. 34), the essentials
of the process of syngamy are as before — the fusion

FIG. 34.

Illustrating the process of syngamy in Stylorhynchus (after

Leger).

$ . Male gamete ; $ , female gamete.

together of two cell-individuals (gametes) to form a single
individual (zygote) — but in this case the two gametes are
no longer exactly alike : one is a rounded motionless
creature (i, $) ; the other is somewhat spindle-shaped,
one end being prolonged into a powerful flagellum, and
swims actively hither and thither, until coming into the
neighbourhood of a gamete of the first type it is attracted

REPRODUCTIVE PROCESS IN ANIMALS 59

to it and becomes completely fused with it to form the
zygote (Fig. 34 (1-4)).

In the case of Plasmodium (Fig. 35), the difference
between the two gametes which undergo syngamy is
still more marked. The one ( $ ) is as before rounded and
motionless, but it is also much larger in size owing to the
fact that its body is distended by granules of condensed
food-material which it has stored up in its protoplasm.
The other gamete is slender, very much smaller, and swims

FIG. 35.

Gametes of Plasmodium, the malaria microbe.
£, Male gamete ; $ female gamete.

with great rapidity until passing near a gamete of the first
type it is drawn towards it by some apparently irresistible
attraction and becomes merged in it to form the zygote.

In Plasmodium we find established the conspicuous
sexual differences in size and appearance which are
characteristic of the gametes as a general rule throughout
the Animal Kingdom — differences so marked that the
two types of gamete were formerly regarded as things
essentially different in nature and given distinct names-
ova and spermatozoa. Further, we can recognise other
features characteristic of maleness and femaleness in
general — the active movements and roving disposition
of the male ; the relative inactivity, stay-at-homeness,
of the female, her fatal attractiveness to the male, her
tendency to hoard food.

60 SEX AND HEREDITY

In the life-history of a simple Protozoon like Copro-
monas we have seen that there are associated together
two very distinct processes — one of reproduction, in which
the individual becomes resolved into two new individuals,
and the other — syngamy — in which two individuals
become merged into one. The same is the case amongst
these lowly organized creatures in general : the zygote-
an individual formed by the fusion of two gametes-
reproduces by the simple process of fission over and over
again ; then syngamy takes place and the new zygote
goes on reproducing as before — and so on indefinitely.

The process of syngamy appears as a general rule to
be an essential part of the cycle. If a single individual
be isolated in a vessel by itself, and kept under suitable
conditions as to food supply and so on, it will go on
reproducing for it may be some hundreds of generations.
From time to time, however, waves of depression seem
to pass over the culture, during which the reproductive
activity becomes slackened. As time goes on these
waves of depression become more and more marked : the
vitality of the individuals becomes obviously impaired ;
they degenerate and undergo a kind of senile decay, and
eventually the whole culture dies off.

Now it has been found that it is possible to tide a
culture successfully through these periods of depression
by making some marked change in the conditions under
which the culture is living, or by applying some special
stimulus. When this is done the individuals appear
to renew their vitality, and proceed to reproduce over and
over again as before.

Here then we learn a very important lesson. We
have been accustomed to regard Death as a necessary
sequel to Life, but we see that amongst these lowly

REPRODUCTIVE PROCESS IN ANIMALS 61

organisms this is not so. The living substance that
constitutes their body is potentially immortal : provided
it gets an appropriate stimulus as a corrective to the
period of depression it does not die, but simply divides
into two and goes on living as before.

Now in Nature such a stimulus exists normally in the
process of syngamy. If instead of the descendants of
one zygote being kept in a vessel by themselves two
distinct broods are mixed together there comes a time
when the individuals of the two broods become gametes,
each fusing with one of the other brood to form a zygote,
and each zygote with its new lease of life proceeds to
divide over and over again as before.

There is one other point we should notice before leaving
these lowly organized Protozoa. It will be remembered
that the zygote of Copromonas normally went through
a resting period enclosed in a shell or cyst (Fig. 33 (n) ).
The use of this is clearly protective — to shield the living
zygote from unfavourable external conditions. Now,
such a process of encystment is very generally associated
with the zygote stage of a Protozoan life-history. And,
as we might expect from this, there is often an obvious
tendency for the process of syngamy to be associated
with some unfavourable change in external conditions-
such as the drying up of a pool, the absence of food, the
coming on of winter. It will be understood that the
onset of ' unfavourable conditions ' may consist either
of actual alteration in the external conditions themselves,
or of alteration on the part of the living organism itself,
so that it gets in some way ' out of joint with its
surroundings.

We may take it as a general principle that the process
of syngamy tends to be associated with alteration in the

62 SEX AND HEREDITY

normal relations between the protozoon and the sur-
rounding world — whether the actual change takes place
in the circumstances of the outer world or in the vital
activities of the creature itself.

If we turn from the Protozoa to the more complicated
types of animal we find processes taking place which are
in their essence the same as those which we have studied
in the Protozoa. Any one of these more complicated
animals — say a Lobster, a Fowl, or a Man — begins its
existence as a single cell — a zygote formed by the fusion
together of two gametes. Then there follows a process
of fission repeated over and over again — the zygote
dividing into two cells, each of these dividing again and
again, and so on for hundreds or thousands of generations.

But there is this striking difference from the Protozoon.
In the latter, when the zygote divided into two, the two
cells so formed separated, swam away, and lived their
own lives as independent individuals. In the animals
higher in the scale however, the cells remain attached
together so that we get a coherent mass of cells, 2, 4, 8,
16, 32, 64, 128, and so on. This mass of cells becomes
larger and larger with successive cell-divisions — it grows
-it is the body of the individual — it goes on growing
until at last cell-division slackens off and the animal
attains a more or less definite adult size.

Just as a large community of civilized human beings,
such as a great city, requires an enormously complicated
organization to provide for its various needs, as compared
with a savage community composed of a few almost
independent individuals, so this immense community of
cells which constitutes the body of one of the higher
animals has to undergo an extraordinarily complicated
process of organization. Certain tracts within its living

REPRODUCTIVE PROCESS IN ANIMALS 63

substance become hard and stiff to form a framework
to support the soft protoplasm — the skeleton. Masses
of cells become developed into contractile muscles for
pulling about the various parts of the skeleton, and
consequent movement of the body as a whole. Other
tracts of cells have to do with digesting and absorbing
the food : others with the getting rid of poisonous waste
materials. Others become developed into an elaborate
transport system — the blood — which distributes the food
material throughout the body and collects the waste
material ; still others into that marvellous nervous
system which has to do with receiving impressions from
the external world, with linking the various parts of the
body together and controlling their activities, and with
that wonderful process which we call thinking.

The myriads of cells which constitute the adult body
become highly specialized foi their various walks in life.
But this specialization brings in its train the loss of that
great primitive power — the powrer of undergoing fusion
together — syngamy — with its accompanying drinking in of
that elixir of life which renews their vitality and enables
them to continue alive. And so it is that the body of
these higher creatures is doomed to suffer unavoidable
natural death. Whereas the living substance of the
Protozoon is potentially immortal — provided that circum-
stances remain favourable and that from time to time
it unites in syngamy with other living substance, it may
go on living indefinitely — the higher animal has on the
other hand its days upon the earth numbered, however
favourable may be the conditions under which it exists.

But — and here is one of the most fascinating features
of animal organization — there lurk somewhere or other
in its body one or more clumps of cells which have not

64

SEX AND HEREDITY

3
T)

U

o

undergone this fatal specialization, which have not lost

their power of undergoing syngamy,
and which therefore have not lost their
potential immortality. These cells
constitute what is called the gonad-
the mass of reproductive cells — while
the rest of the body is known as the
soma. It is only the soma whose days
on the earth are numbered — any one
of the reproductive cells if allowed to
undergo syngamy receives in this pro-
cess its new lease of life, just as was the
case with the Protozoon, and proceeds
with the repeated sub-divisions which
build up the body of a new individual.
The gonad or mass of reproductive
cells lives within the soma or main part
of the body. It is in it, but yet not
of it. It lives its own life, protected,
nourished, and carried about by the
soma. While the gonad is not mortal
in the sense the soma is — not necessarily
ending its existence in natural death-

those parts of it which remain within
the body are subject to a violent and
so to speak accidental death if anything
happens to the soma upon which it is
dependent for food and so on.

An important achievement of modern
research has been the proof — in the
case of certain animals — that the soma
is, as it were, " side-tracked ' from the gonad at an
extremely early stage. In some cases indeed when

- f. 5

-r .a •z

H >

•

REPRODUCTIVE PROCESS IN ANIMALS 65

the zygote undergoes its very first fission into two cells
one of the two is seen to be already marked off as the
somatic cell from the other which remains as gonad.
The gonad is in fact simply a set aside portion of the
substance of the zygote, in other words a persisting
portion of the gonads of the two parents.

The fact that the gonad is not produced by the indi-
vidual in whose body it lies, but is rather a persisting
portion of living substance handed on from the previous
generation and living its own life within the surrounding
soma, is of great practical importance, for it renders less
mysterious the fact that what are called acquired characters,
or better impressed characters, are not inherited. If the
soma or body has any striking modification impressed
upon it during its lifetime — say a scar produced by
disease or injury, or the loss of a limb — this feature is
not handed on to the next generation. It is one of the
few comforting reflections during this horrible war, when
we so often see those dear to us maimed for life, that at
least these injuries do not register themselves in the
gonad so as to be passed on to the generation to come.
And this fact ceases to be surprising when we remember
that the gonad is not produced by the individual in whose
body it is contained, but is a heritage from the generations
that have gone before.

S.H. E

LECTURE IV

SOME OF THE MODIFICATIONS OF THE REPRODUC-
TIVE PROCESS AS ADAPTATIONS TO LIFE

UPON LAND

As was the case with the Vegetable Kingdom, all the
evidence goes to show that animals were originally
inhabitants of the water. However, as Evolution has
proceeded, many different groups of animals have taken
to the land, being enabled to survive there by various
interesting modifications. The most conspicuous of these
is the development all over the surface of the body of
an impermeable layer to prevent the drying up of its
substance — for living protoplasm has been unable to
accustom itself to drying up. It may probably be said
truly that ' Dry protoplasm is dead protoplasm." The
impervious outer coating of the land animal fulfils then
an important function in retaining the moisture within
the body.

*>

Enclosed within this covering is the cell community-
these myriads of highly specialized and actively co-
operating cells that constitute the living body. Each of
these has its surface in contact with the watery fluid
which everywhere permeates the body. The individual
cells lives in this fluid just as a simple protozoan animal
lives in its pool or pond ; and Nature has adapted the

66

MODIFICATION OF REPRODUCTIVE PROCESS 67

cell to live in this medium just as she has adapted the
inhabitant of the pool or pond to its own watery medium.
This internal medium of the animal body however is
not pure water but a very complicated mixture. The
incessant living activity of the protoplasm causes constant
formation of waste materials, and these are discharged
into the fluid. Now these waste products are by no
means the same : they vary in different kinds of cells
and in different organs — each of which contributes its
quota to the mixture. If any particular organ omits
its contribution, or if its contribution is abnormal in
quantity or quality, then the composition of the internal
medium is altered and the health of the whole body is
liable to suffer, owing to the fact that its cells are
adapted to live in the medium of a certain ' normal '
composition.

One of the chief obstacles wrhich have lain in the way
of animals becoming adapted to life on land has had
to do with the early stages in their life-history. For
these early stages repeat in a somewhat crude form
earlier stages of evolution, in which the habit was purely
aquatic. Thus if one examines an early stage in the
development of one of the higher Vertebrates — say a
fowl, or a human being — one finds along the sides of
its neck gill-openings such as those of a fish, although the
fowl or man will never have occasion to use them for
breathing ; again the main blood-vessels are arranged
on the same plan as those of a Fish for the conveying of
blood to and from the gills ; again the skeleton is of a
simple gristly nature like that of one of the lower fishes ;
and so on with various other organs of the body. Any
zoologist finding such a creature living free in Nature
instead of within the egg-shell or the body of the parent

68

SEX AND HEREDITY

would at once classify it with the fishes. It is in fact a
Fish stage in development.

It will readily be understood what a serious difficulty
the existence of such aquatic fish-like stages of develop-
ment has constituted in the wray of the assumption of a
purely terrestrial habit. The present lecture will deal

with the methods by which the group
of animals to which we ourselves
belong — the Vertebrates — have over-
come this particular difficulty.

\Ye may commence with the Am-
phibians— the group of Vertebrates
which includes Frogs and Toads — a
group which is of special interest from
the fact that it has not succeeded in
emancipating itself entirely from the
ancestral watery environment, but
yet has made a number of very
interesting attempts in this direction.
In the early spring one may see in
ponds and ditches masses of spawn
of the ordinary frog — the eggs or
zygotes having the appearance, as
seen from above, of little black
spheres about -^V inch in diameter, each enclosed in a
larger sphere of clear crystal jelly. In due course the
eggs develop into tadpoles, which represent the fish stage
of development. In the case then of the ordinary frog
or toad, although the adult has emancipated itself from
the water to a certain extent — it is able to live on land
though it needs a moist atmosphere — the early stages of
its life-history are still purely aquatic.

In an interesting Tree-frog called Phyllomednsa, which

FIG. 37.

Phyllomedusa sauvagii, mass
of spawn enclosed between
leaves. (From Graham Kerr's
Embryology, after Agar.)

MODIFICATION OF REPRODUCTIVE PROCESS 69

Budget! and Agar studied in South America, the spawn
is deposited between the leaves of plants overhanging the
edges of pools. Here it hangs while the eggs undergo
the early stages of their development, but when the Tad-
pole stage is reached a kind of digestive juice is secreted
which causes the jelly round the eggs to soften and
liquefy and trickle down into the pool, carrying the

FIG. 38.
Alytes obstetr leans, male carrying eggs. (After Boulenger.)

tadpoles with it. Here then the tadpole has to pass the
later part of its existence in the water, but all the earlier
stages are independent of the water.

A somewhat similar case is that of a Japanese Tree-
frog (Rhacophorus schlegeli), where the mass of spawn is
deposited in a burrow excavated in a bank of earth, by
the margin of standing water. After depositing their
spawn the parents make their way out by a tunnel sloping
downwards towards the water's surface, and when the
appropriate stage of development has been reached the
jelly becomes liquefied and trickles, with its contained
tadpoles, down this tunnel into the water.

70 SEX AND HEREDITY

In the most interesting attempts on the part of frogs
and toads to free their life-history from the aquatic
environment, the eggs or young are carried about by the
parent. In the case of Alytes, a Toad common on the

FIG. 39.

Male of Phyllobates tnnitatis, carrying Tadpoles. (From Graham Kerr's
Embryology, after Boulenger.)

continent of Europe, the eggs are laid on land, and the
male parent carries them about in a mass, composed
really of two bead-like strings, attached round his legs.
At intervals he visits a pool of water and the eggs are
moistened ; eventually, on one of these visits, the tadpoles
hatch out and thereafter lead a normal aquatic existence.

FIG. 40.

Female of Hyla gceldii, carrying eggs. (From Graham Kerr's Embryology, after

Boulenger.)

In various other cases the tadpoles j ourney on land from
one pool to another, hanging on to the back of the male
parent (Fig. 39). In a Brazilian Tree-frog — Hyla goeldii
— the eggs are placed on the back of the female, and their

MODIFICATION OF REPRODUCTIVE PROCESS 71

presence there causes, in some mysterious fashion, the
skin to grow up into a ledge all round, forming a kind
of saucer-shaped receptacle in which the eggs are borne
about (Fig. 40). In another South- American Frog-
Nototrema (Fig. 41) — the same thing happens, but in
this case the rim of the saucer grows more actively on
each side and turns inwards, so that eventually the two
edges meet and form a roof over the eggs, which thus
come to be contained in a deep pouch, opening by a
narrow slit which may be further reduced to a small
pore at its hind end.

Then there is the extraordinary Surinam Toad — Pipa.
In this case also the eggs are deposited on the back of the
female, spaced out at intervals, and their presence causes
the skin in their neighbourhood to grow up round the
eggs so that each one comes to be enclosed in a deep pit
or cell which is closed in by a closely- fitting lid. Each
in its narrow cell, the young toads proceed with their
development, passing through the Tadpole stage before
eventually they emerge to lead their independent existence
(Fig. 42).

Lastly may be mentioned the little South- American
toad, Rhinoderma darwini, in which the male parent
swallows the eggs into the croaking-sac or sounding
chamber which lies under the skin of the breast, and in
the safe seclusion of which the young toads proceed
with their development.

It will probably be admitted by everyone that the
cases of these Frogs and Toads constitute an extra-
ordinarily interesting series of attempts to get rid of the
free aquatic existence during the early stages of the life-
history.

It is only, however, when \ve come to the Reptiles and

72 SEX AND HEREDITY

Birds that we find complete emancipation from the watery
environment. The successful attainment to this has
been rendered possible partly by the development of a
horny impermeable skin in the adult, and parti y by
modifications in the reproductive processes. It is the
latter alone which concern us now.

Fro. 41.

Nototrema pys.ma.eum, female. The eggs, large in size and few in number in this
species, are seen showing through the roof of the pouch in which they are contained.

In the Bird the female gamete — or macrogamete, or
egg — is of relatively enormous size — a spherical cell,
packed with reserve food-material or yolk, food-hoarding
having here reached its maximum. It is what in domestic
language is called the yellow or ' yolk ' in the case of
the Hen's egg.

The act of syngamy — the fusion together of male and
female gamete — takes place within the body of the bird,

MODIFICATION OF REPRODUCTIVE PROCESS 73

and the zygote so produced slowly travels along the
tubular passage (oviduct) towards the outside. As it
does so the lining of the tube deposits on the surface of
the zygote layers of covering material — first the white
or albumen (Fig. 43, alb), with thicker strands (ch)
towards each end to moor the zygote in position, then a

FIG. 42.
Pipa amcricana, female with young escaping from the cells upon her back.

tough white membrane (s.m), and lastly, as the egg
approaches the exterior, a layer of limy material which
solidifies to form the hard porous shell. The membrane
is split into two layers towards the broad end of the
shell — the end which is directed towards the external
opening of the oviduct as the egg travels along — and
after the egg is laid, as the white gradually shrinks in
volume, air accumulates in the space between these two
layers (Fig. 43, a, s). Hence it is that when an egg not
quite fresh is placed in water the broad end instantly

74 SEX AND HEREDITY

bobs upwards. The air-space is of importance to the
young bird, for when, just before hatching, it begins
to struggle within the egg-shell, its beak penetrates the
air space, it takes its first breath of air, and so invigorated
the young chick is able to break the shell and step into
the world which lies outside.

The egg-shell varies much in different birds. Where it
is freely exposed to view and to daylight it often shows
a beautiful protective colouring which renders it very

FIG. 43.

View of a Hen's egg, freshly laid, with part of the shell broken away so as to
expose the contents.

The " yolk " or true egg is seen in the centre with the whitish protoplasmic
portion (blastoderm) uppermost, a.s, air space ; alb, albumen or white ; ch,
denser strand of albumen towards each end ; s.m, shell-membrane. (From
Graham Kerr's Embryology.)

inconspicuous, as for example in the case of the Peewit
and other kinds of Plover. Its shape also varies : it is
often pointed at one end, moulded within the parental
body while still soft by the squeezing pressure of the
oviduct forcing it onwards. And Nature has exaggerated
this pointed shape in some of the eggs which are laid
on shelves of rock, such as those of the Razorbill or
Guillemot, so that when it gets a knock the egg merely
runs round without rolling for any distance.

The young bird shows many interesting developments
while it is contained within the egg-shell. The yolk

MODIFICATION OF REPRODUCTIVE PROCESS 75

provides it with an ample supply of food, so that it does
not have to get into the outer world and fend for itself
until a very late stage in its development. Thus a watery
environment is no longer necessary for the early fish-like

FIG. 44.

Diagram illustrating the contents of a Hen's egg which has been incubated for
twelve days. The young bird is seen within the cavity of the amnion. Attached
to its lower side are seen two stalks — -the hinder one connected with the allantois
(all) which lines the shell, the other with the yolk-sac in which the yolk (y) is con-
tained, alb, remains of albumen or white. (From Graham Kerr's Embryologv,
after a figure by Lillie in his Development of the Chick.)

stages, for these are passed through in the interior of the
egg-shell.

Then the body of the young bird comes to be enclosed
in a thin bag filled with watery fluid, and known as the
Amnion (see Fig. 44), which forms a water-jacket to pro-
tect its delicate substance from the jars to which its being
on land exposes it.

Again, a bladder-like organ — the Allantois (Figs. 44
and 45, all) — bulges out from its body and flattens itself

76

SEX AND HEREDITY

out against the inner surface of the shell, gradually lining
the whole of it and serving as the breathing organ by
which the young bird absorbs the oxygen necessary to
its life, through the pores of the shell. The remains of
the allantois may be seen as a thin membrane containing

10

Ctt

all
y.s.

FIG. 45.

Young fowl extracted from an egg which has been incubated for 4^ days.
all, allantois ; C.H, cerebral hemispheres of brain ; E, eye ; som, edge of opening
through which the allantois and yolk-sac project ; t.o, optic lobes of brain ; V,
In-art ; v.c, gill-openings; y.s, yolk-sac. (From Graham Kerr's Embryology

a rich network of bloodvessels lining the pieces of shell
from which a young bird has hatched.

The Mammals — those animals which possess hair and
feed their young on milk — constitute the group to which
we ourselves belong, and so we speak and write of them
as the highest of living things — just as no doubt an Ant
or a Cuttlefish, were it able to write books, would place
in this position the Insects or the Molluscs !

MODIFICATION OF REPRODUCTIVE PROCESS 77

In these animals we find the modifications of the repro-
ductive processes to fit a purely terrestrial existence at
their highest level. Right down at the bottom of the
scale of mammals we find two very ancient creatures
still surviving in Australia and New Guinea (Echidna and
Ornithorhynchus) in which the eggs are large, provided
with a supply of yolk as in the case of Reptiles and
Birds, and are actually laid — in the case of Onritho-
rhynchus being deposited in an underground burrow,
while in that of Echidna they are carried about by the
mother in a specially developed pouch on the lower
surface of the body.

In the next phase of evolution, represented by the
Kangaroos and other Australian mammals and by the
American Opossums, the eggs are hatched while still
inside the mother's body, so that instead of eggs being
laid, young are born. These young are very helpless and
inperfectly developed, so the mother carries them about
for some time, either hanging on to her body or contained
in a pouch specially developed for the purpose, until they
are able to look after themselves.

In the ordinary mammals a step further has been
taken, the young animal being retained within the
mother's body for a much longer period, until a much
higher degree of development has been reached. In this
way not only are the early stages which should be aquatic
spared the need of a watery environment but they remain
within the body and, as it wrere, part of the body of a
fully developed adult creature, with all its capacity
developed for looking after itself in the struggle for
existence.

Here again we find very interesting arrangements
adapting the embryo to its life within the body of the

78 SEX AND HEREDITY

adult. As the young animal is able to absorb food from
the blood of the mother it is no longer necessary to have
a reserve supply of food-material or yolk stored up within
the egg : hence we find that the egg of the mammal has
reverted to the condition of a very minute cell, a simple
sphere of protoplasm containing a nucleus, measuring
perhaps the yj^ of an inch (Man, see Fig. 2, B) — in
striking contrast with the relatively huge egg of the
Reptile or Bird.

Again, as the egg is to develop within the body of the
mother, elaborate protective coverings, such as those
seen in the case of the Fowl's egg, are no longer necessary
and have disappeared. The amnion is present as before.
So also is the allantois, but this has undergone a great
increase in complexity. It comes into close contact with
the lining of the enlarged oviduct or uterus. Its surface
is covered with a thick layer of protoplasm which fits
itself close to the uterine surface, insinuating itself into
every little crevice, and finally eating its \vay into the
wall of the uterus and spreading along the course of the
blood-vessels in this wall — blood-vessels belonging to
the mother. In the layer of protoplasm other blood-
vessels develop, belonging to the embryo.

There thus come to be associated together two sets of
blood-vessels — through one of which courses blood be-
longing to the mother and through the other blood
belonging to the embryo. For a time these are separated
by a considerable thickness of the protoplasm covering
the allantois, but this gradually disappears and there is
nothing left between the two blood-streams but an
extremely thin, though unbroken, membrane walling in
the vessels of the embryo. Through this thin membrane
there diffuse into the blood of the embryo from that of

MODIFICATION OF REPRODUCTIVE PROCESS 79

the mother, (i) food-material for its nourishment and
(2) the oxygen needed for its living activity. In the
opposite direction there pass away from the blood of the
embryo into that of the mother the various waste materials
produced by the vital processes of the embryo. This
arrangement of interlocking blood-vessels, by which
interchange takes place between the blood of the mother
and of the unborn young, constitutes the greater part of
a very complicated organ known as the placenta, or in
the case of man as the ' after-birth," from the fact
that it is shed and got rid of soon after the birth of
the child.

The young individual leads its pre-natal life in the
comparative safety and seclusion of the uterus, hanging
on to its wall as a parasite, its needs being ministered to
by the activities of the placenta. But it is already, from
the zygote stage on, a new self-contained individual,
possessing, though it may be in a latent form, all the
normal characteristics of its race. So long as the mother
remains in a normal state of health the young individual,
when once it has come into existence by the act of
syngamy, does not appear to be affected by her special
peculiarities. Thus it has been found possible to take
the eggs at an early stage of their development from the
uterus of a Rabbit belonging to a particular breed (Angora)
and transfer them to the uterus of a Rabbit of a quite
different breed (Belgian Hare). The eggs so transferred
went on with their development, and the young rabbits
when born were found to be perfectly typical (Angora),
showing no signs whatever of having been influenced in
any way by the peculiarities of the foster-mother in whose
uterus they had sojourned during almost the whole period
of their development.

So SEX AND HEREDITY

This comparative independence, on the part of the young
individual, of the peculiarities of the mother, holds under
normal conditions. If, however, conditions become
abnormal through disease, in particular if the blood-
stream of the mother, upon which the welfare of the young
individual is completely dependent, becomes abnormal,
then there are apt to come about effects upon the health
of the young individual which may be disastrous.
Poisonous substances in the blood of the mother diffuse
into the blood of the embryo and poison it. And various
disease-producing microbes have the power of burrowing
their way through the thin membrane separating the
blood streams of mother and offspring, and thus infecting
with the disease the new individual.

LECTURE V

HEREDITY

IN this lecture we have to consider the factors at work
which determine the degree of similarity between parent
and offspring, or between the various offspring of the same
parents. The problem is fundamentally the same through-
out the whole animal and vegetable kingdoms, though
simplified in degree in the case of the lowest, unicellular,
organisms. In what follows we shall neglect these and
confine ourselves to the higher organisms, where the
problem is conditioned by two main factors.

Firstly, that the bridge between two generations is the
minute germ-cell or gamete which, being unicellular, is of
a different order of structure from either the parent from
whom it was derived, or the individual into which it
develops.

Secondly, that each individual of the new generation
is usually produced by the fusion of two cells (gametes)
derived from different individuals of the parent generation.

The fact that the bridge between parent and offspring
is the minute unicellular gamete at once raises the question
of what is the physical substratum on which heredity
depends. Is there any part of the gamete which can be
recognised as presenting those properties which we
should expect this substratum to possess ?

S.H. 8 1 F

82 SEX AND HEREDITY

This question can best be answered by considering a
concrete case of syngamy and early development of
an animal, and we will choose as our example the
little, almost microscopic, shrimp-like creature Cyclops.

Fig. 46, A, shows the egg or macrogamete of Cyclops
immediately after the entry of the spermatozoon (m), or
microgamete. The great bulk of the egg consists of
protoplasm densely packed with yolk. Like all cells,
the egg contains a nucleus (/), which is seen close under
the egg shell. It has travelled to this position prior to
dividing into two and getting lid of one of the halves.
This at once introduces us to one of the most important
features of the hereditary substance. If, as we are
bound to assume, this substance has a definite structure,
it is obvious that there must be some method of reducing
it by half once in each generation, since otherwise it would
be doubled by the fusion of gametes in each act of syngamy.
Now we find that a halving of the nuclear material of
the gametes takes place at or before syngamy (in the case
of the spermatozoon this halving has already taken place
before it enters the egg) . This fact in itself points there-
fore to a probability of the nuclear substance being the
material substratum of heredity.

In Fig. 46, B, we see both the male and female nuclei
travelling in towards the centre of the egg, the nuclear
material (p) discarded from the female nucleus being
conspicuous just under the egg shell. It can be seen in
this position for a long time in development, but itself
plays no part in this process, being indeed dead and
rejected.

Fig. 46, C, shows the two nuclei (m and/) closely applied
to one another in the centre of the egg, and we notice the
important fact that they are now equal in size. Fig.

HEREDITY

f.

FIG. 46.

Syngamy and early development in the Crustacean, Cyclops (diagrammatized
from Hacker and Amma) ; /, nucleus of macrogamete or egg ; g, primitive germ
cells ; in, nucleus of microgamete or spermatozoon ; p, rejected portion of female
nucleus ; A, immediately after entry of microgamete into the egg, female nucleus
dividing into two ; B, having thrown out half of its substance, (p) the female
nucleus is moving in towards the centre of the egg to meet the male nucleus ;
C, the male and female nuclei, now alike in size and constitution, have met in the
centre of the egg to form the zygote nucleus ; D, first division of the zygote
nucleus. The six chromosomes (three derived from each gamete) have each
divided into two V-shaped bodies ; E, two sets of six chromosomes separating.
Division of the cell body beginning ; F, division complete, the new nuclei recon-
stituted and the egg divided into two cells ; G, the zygote nucleus in C shown under
a higher magnification. Three chromosomes, in the form of long beaded threads
are seen in each portion ; H, 1-5, stages in the division of a single chromosome ;
/, a dividing nucleus (as in D), at a higher magnification ; K, a later stage of
development. The two cells with large nuclei, and granules in the cell body
are the primitive germ-cells of the young Cyclops ; L, a much later stage, at a
lower magnification. Stomach, muscles, etc., now developed.

84 SEX AND HEREDITY

46, A, shows the enormous disparity in size between the
microgamete, which consists almost wholly of nucleus,
and the macrogamete. In many animals this disparity is
very much greater, culminating in the birds with their
enormous eggs. In the Ostrich it can be calculated that
the macrogamete (yolk of the egg) is many billions of
times as bulky as the microgamete. The amount of
nuclear material in the two gametes is however equal,
though owing to the fact that this is in a very concentrated
state in the microgamete it may appear less bulky here.
As a comparison between Figs. 46, A, and 46, C, shows
however, by the time the nucleus of the microgamete has
loosened out to the same texture as the female nucleus,
the two nuclei are the same size.

V

Now it is a matter of general observation that in-
heritance throughout the animal and vegetable kingdoms
is on the average as strong from the male as from the
female parent, and so it is reasonable to suppose that the
physical substratum on which inheritance depends is
provided in equal quantities by the two parents. Hence
we are again led to look to the nuclear material as probably
the substance of which we are in search.

The development of an animal (or plant) from the
single celled zygote consists of two main processes :

(1) The division of this cell into a great number of
cells.

(2) The differentiation of these cells into the various
kinds of cells — skin cells, muscle cells, nerve cells, etc.-
which compose the body. We will follow the first of
these processes for a short distance in the development
of Cyclops.

The division of the zygote into the first two cells of the
embryo or young Cyclops is shown in Figs. 46, D, E, F.

HEREDITY 85

In all cases, division of the cell as a whole is preceded
by the division of its nucleus. In Fig. 46, C, we see that
each gamete nucleus (shown on a larger scale in Fig.
46, G), contains three long threads, the combined zygote
nucleus therefore containing six. These threads are
composed mainly of a substance called chromatin, and
each of them is called a chromosome. The first step in
the division of the nucleus consists in each chromosome
splitting along its length into two. At about the same
time fine fibres (shown in Fig. 46, D, etc.), make their
appearance in the protoplasm, stretching from points at
opposite poles of the cell. Of the two chromosomes pro-
duced by the splitting of each original one, one becomes
attached to a fibre running to one pole, and the other to
a fibre running to the opposite pole of the cell. The
chromosomes then travel up the lines of these fibres to
congregate at the poles, where they form two new nuclei,
one at each pole. Thus it will be seen that each gamete
introduces three chromosomes, providing the zygote
with six, and that each of the two nuclei formed by the
division of the zygote nucleus gets a half of each one of
these six chromosomes. In other words, the chromatin
of each nucleus is derived in equal quantity from the male
and female parent.

The developing egg, or embryo as we may now call it,
now consists of two cells, each with its nucleus. By a
process similar to that just described each nucleus again
divides into two, followed by the division of its cell, so
as to give four cells, and this process is repeated again
and again until a large number of cells, each with a
nucleus, has been formed. Thus the nuclei of all the
cells of the embryo and adult Cyclops contain chromatin-
or hereditary substance as we may now call it — derived

86 SEX AND HEREDITY

in equal amounts from the two parents. In later stages,
it is true, the double appearance of the nuclei, indicating
their origin from the two gametes, disappears (Fig. 46, K),
owing to the two sets of chromosomes having mingled
together. The nuclei, however, still contain the six
chromosomes, which become visible in certain phases.

Figs. 46, H-J, show the nucleus and the division of
the chromosomes on a larger scale. It can be seen that
the chromosomes are not homogeneous, but that each
consists of a row of chromatin granules like a string of
beads. Fig. 46, H, 1-5, illustrates in detail the process
of division of a chromosome, and shows that the splitting
of the chromosome as a wrhole consists in the division
of each one of these beads so that two parallel
rows of beads are formed. Later, the chromosomes
contract so much that the individual beads are no longer
visible — hence they are not seen in Fig. 46, D, which is
the stage shown in Fig. 46, /, on a larger scale.

These chromatin beads are of course living and
growing bodies, and hence the halving of the size of each
bead which occurs in each division is compensated by
growth in the intervals between divisions.

This mode of development, and of distribution of the
chromatin, is of general occurrence in the animal and
vegetable kingdom, though the number of chromosomes
varies in different species. The general rule obtains,
however, that the gamete has half the number found
in the zygote, the former having thrown out half of its
chromosomes, as we have seen in Cyclops, in preparation
for syngamy.

Summing up, we are led to identify the chromatin, or
some substance intimately bound up with the chromatin,

HEREDITY 87

of vthe nucleus with the material substance on which in-

E

heritance depends for the following, among other, reasons.

(1) Equality in quantity of chromatin in the two
gametes, in spite of their enormous disparity in size as
a whole.

(2) Accurate division of each element of this material
at each nuclear division, so that every cell in the body
gets a derivative of all the chromatin elements of both
gametes.

(3) Reduction of the amount of chromatin to one half
in the gametes, so that doubling is avoided when they
unite.

With this knowledge of the physical basis of inheri-
tance, we must proceed to consider the organism as
forming one of an endless series of generations.

Perhaps the easiest way to grasp the point of view
from which modern biologists regard the organism as a
link in the chain of ascendants and descendants is to com-
pare the two theories of heredity propounded, the one by
Darwin and the other by Gait on. With fuller knowledge,
Darwin's hypothesis has had to be rejected. (It must not
of course be supposed that we are referring in any way
to his great work on Evolution by Natural Selection.)

Darwin proceeded from what may be called the natural
or common sense way of looking at the organism. He
and his contemporaries were exercised as to howr the
characters of the parent got into the gamete. In break-
ing into the endless chain of parent-germ-parent (the old
problem : which came first, the chicken or the egg ?) he
started with the parent, and the problem of heredity
was : how could its characteristics be compressed into the
minute and apparently nearly structureless gamete?

SEX AND HEREDITY

The ( provisional hypothesis ' which Darwin proposed
to account for this was that every cell in the body is
continually throwing off ultra-microscopical particles,
which he called gemmules, and which got into the blood
stream and were thus carried to the reproductive organs
and became stored up in the germ-cells. These latter were
in fact, in his view, nothing more nor less than little
packets of gemmules, and the development of the germ-
cell into the individual of the next generation consisted
in each gemmule growing into a cell like the one which
had produced it.

Galton looked at the stream of life from a different
point of view. He broke into the parent-germ-parent
chain at the germ — or rather he denied the existence of a
chain at all, but looked upon the stream of life as a straight
line of germ-cells, giving off blind side alleys in each
generation — the bodies of the organisms which we know.

This viewT was much elaborated by Weismann, to whom
we are indebted for a more detailed conception of the
relation between the germ-plasm and the body-plasm-
the former being of course the protoplasm (more particu-
larly, the nuclear material or chromatin) contained in
the germ-cells, while the body-plasm forms the substance
of the body cells. He pointed out that the germ-plasm
is potentially immortal, that is to say, it does not die
provided that its environment is of a suitable nature,
whilst the body-plasm is mortal.

To a certain extent this proposition may be described
as a truism, since it is of course obvious that the living
substance of which all of us are composed is continuous
back to the beginnings of life on this globe, millions of
years ago. Such part of that living substance wrhich has
entered into our bodily structure, however, is doomed

HEREDITY 89

to perish, as did the vast array of bodies produced in the
past by our own particular streams of germ-plasm. The
germ-plasm alone endures, and may continue to endure
for an indefinite number of millions of years, giving off
further innumerable mortal bodies.

This conception of the relations between germ-plasm
and body-plasm is well illustrated by the early develop-
ment of Cyclops. In Fig. 46, C, we see at each side of
the nucleus a number of radiating lines, the first appear-
ance of a sort of anchor in which the fibres which guide
the movements of the chromosomes will later be fixed
(Fig. 46, D). It is to be noticed that round about one
of these radiations are a number of dark granules, while
they are absent from the radiation at the opposite pole
of the nucleus. This results in one of the two cells formed
by the first division of the zygote possessing these granules,
while the other one is without them (Fig. 46, E, F).

The next stage in development is that each of these
two cells with their contained nuclei divide into two, in
exactly the same way as before. The two cells derived
from the cell which lacks the granules will of course have
no granules. In the case of the other cell, the granules
behave as before, all congregating at the one pole of
the dividing nucleus, leaving the opposite pole free from
them. Division therefore results as before in one cell
with granules and the other without. Thus in the four
cell stage of the embryo we find three cells without
granules and one with them. For a time cell multipli-
cation proceeds in the same manner, all the descendants
of the cells without granules being free from them, while
the cell containing the granules always divides into one
cell with them and one without. The embryo thus
always has one cell, and only one, containing granules.

90 SEX AND HEREDITY

This mode of development continues till the embryo
consists of sixty-one cells, of which therefore sixty are
without the granules and one contains them. Before
the next division of the granule cell, however, the granules
instead of concentrating at one pole, scatter through the
cell, so that both products of division contain granules,
and the embryo has now two granule cells. From these
two cells arise the germ-cells of the young Cyclops — and
therefore eventually the gametes of the adult. They are
shown in Fig. 46, K, and again in the much more advanced
embryo in Fig. 46, L, where the main features of the fully
formed animal (skin, muscles, stomach, etc.) are already
apparent.

We see therefore that in Cyclops the relations of the
germ-cells to the body is quite in accordance with the
Galton-Weismann view. The body does not manu-
facture its gametes as, for instance, it secretes bile or
saliva. On the contrary, at the beginning of each new
generation the germ-plasm divides into two portions,
one (destined to perish) to form the body of the organism,
the other to lie dormant, enclosed in the body and fed
and protected by it, till the proper time comes for it to
break away from it in the form of gametes and continue
the stream of life.

It should perhaps be mentioned that nothing is known
of the nature of the granules which thus make visible
the distinction between body-plasm and germ-plasm in
Cyclops. They are probably unimportant in them-
selves, as they have not been observed in other animals,
in many of which however the germ-plasm is equally
clearly marked out from the body-plasm by other visible
characteristics not found in Cyclops. While ignorant of
the nature of these various distinguishing marks we can

HEREDITY 91

nevertheless avail ourselves of the convenient labels thus
offered to us.

The Galton-Weismann view deserves to be emphasised,
because it puts a meaning into the word " inherit " which
is rather different to that usually conveyed by that word-
or to the sense in which it would apply if Darwin's view
were the true one. According to Darwin, parents truly
transmit their characteristics to their offspring (by means
of the gemmules) . According to the modern view, however,
children resemble their parents not, strictly speaking,
because the latter have passed something on to them,
but because both have been produced from the same
germ- plasm. Of course it must be remembered that as
a rule the relations of parent and child to the stream of
germ-plasm are complicated by the fact that the offspring
is the outcome of the fusion of two gametes each from
a different stream of germ-plasm. A pictorial repre-
sentation of the stream of life under these normal con-
ditions, compared with the simple state of affairs where
reproduction is carried out without union of the sexes
(as for example in the greenfly, or Aphis], is attempted
in Fig. 47.

The relations between germ-plasm and body-plasm lead
us on to the consideration of the question of the inherit-
ance of ' acquired characters." It is obvious that the
characteristics of an organism are dependent upon two
factors, (i) its original inborn or innate constitution, which
depends upon the germ-plasm (or two germ-plasms) from
which it has sprung, and (2) the particular environment
to which the organism has been exposed and the ex-
periences which it has undergone during its growth and
development. These two factors act with varying effect
in the case of different characteristics. Thus the colour

92 SEX AND HEREDITY

of a man's eye is but little affected by his environment,
but is almost entirely determined by his innate constitu-
tion. On the other hand, the skill of a pianist is plainly
dependent both on his innate mental and physical qualities,
and on the fact that he has been provided with the
opportunity of learning to play the piano.

Fir.. 47.

Diagrams of the stream of life — -A under its simplest condition, as found in
some of the lower animals and plants, where there is only one parent ; B under
ordinary conditions, where two parents are concerned with each act of repro-
duction.

The base lines represent the continuous streams of germ-plasm, the uprights
the bodies produced by them. In A the streams of germ-plasm flow independently,
giving off bo'Jies at intervals. In B bodies are produced only at the intersection
of two streams.

Now if our views as to the relation between individuals
and the germ-plasm which gave rise to them are correct,
only the innate qualities can be inherited, for the germ-
plasm produces the body, not the body the germ-plasm.
For example, take two children of equal (innate) musical
capacity, and put one in a desert island where he will never
see a musical instrument, and train up the other to music
as a profession ; there is no reason to believe that the
children of the professional musician will show greater

HEREDITY 93

musical talent than, given equal opportunity, will those
of the man reared under conditions which prevented him
from ever exercising his talent.

A good deal of confusion exists in the unscientific mind
in regard to the question of the " inheritance of acquired
characters " owing to want of analysis of cause and effect.
It is true, of course, that musical talent runs in families,
but it must be remembered that a man chooses the
profession of music because he has the innate capacity
for music, and it is this innate capacity which is inherited,
not his acquired musical attainments.

We must be careful, however, not to leave the im-
pression that all biologists are agreed that the effects of
environment on the body of the parent can never be
registered in the germ-plasm in such a way as to be repro-
duced in the offspring. There are certain cases which
are still under consideration by biologists. It is also
of course true that acquisitions by the parent may affect
the offspring in other ways, e.g. by the direct transfer-
ence of disease-producing organisms to the embryo, as in
syphilis. Here, however, we are not dealing with inherit-
ance in the biological sense of the word. We may take
it that one runs no practical danger in assuming that
any particular acquisition will not be inherited.

The possibility of predicting the characteristics of the
offspring from those of the parents depends therefore
largely upon how truly the latter disclose the nature of
the germ-plasm from which they themselves have sprung.
Another important requirement is to know what com-
bination will result when two diverse germ-plasms meet.
The greatest advance ever yet made in our understanding
of both these points was the discovery made by a

94

SEX AND HEREDITY

monk, Gregor Mendel, in 1865, though its value was not
appreciated till the beginning of the present century.
Mendel's law, as it is now called, will best be explained
by a concrete example, for which we will choose a case
that has been worked out by Bateson in the domestic
fowl (Fig. 48) .

If a black fowl of the right breed is crossed with a
white l fowl, also of the right breed (top line of the

vvvv

FIG. 48.

Illustrating the inheritance of feather colour in the Andalusian fowl.

diagram), the result is a bird of a slatey blue or gray
colour (second line of the diagram). This bird is what is
known to poultry breeders as the Andalusian fowl. The
result of breeding two Andalusians together is shown in
line three of the diagram. (To save space, in every case
except the first cross, only one parent is shown. It is
to be understood that each bird is mated with one of its
own colour.) Out of every four chickens produced from

1 This breed of fowl is not pure white, but has numerous little
dark points among its plumage. For convenience, however, it
will be called white.

HEREDITY 95

a pair of Andalusians we get, on an average, one black
like the original black of the first cross, two grays or
Andalusians like the immediate parents, and one white
like the original white of the first cross. Breeding from
these again we find (bottom line of diagram) that two
blacks bred together give nothing but blacks, the \vhites
give nothing but whites, while the grays again give all
three kinds in the average proportions of one black, two
grays, one white.

Now the interpretation of this result is as follows. Each
separately inheritable characteristic — such as the colour
of the feathers in the present instance — is supposed to be
represented in the germ-plasm by a definite part or con-
stituent of the hereditary substance, which may conveni-
ently be called a "factor." Thus the germ-plasm of the
white fowl contains the factor for whiteness, and that of
the black fowl the factor for blackness — or as we may
more shortly call them, white and black factors.

When a black and white fowl are crossed, as in the
first line of the diagram, the resulting hybrid has both the
black and white factors, which act together to cause
the gray colour of its feathers. In the nuclei of the germ-
plasm, however, the black and white factors do not mingle,
but may be conceived of as lying separately side by side.
When the gametes are formed — and this is the crux of
' Mendel's law ' -the two factors separate in such a
manner that the number of gametes carrying the black
factor is equal to the number carrying the white. No
gamete, however, can carry both black and white factors.
This process of sorting out of factors in the gametes is
spoken of as segregation.

We are now in a position to understand the result of
mating two gray birds together. The gray hens are

96 SEX AND HEREDITY

producing in equal numbers eggs carrying black factors
and eggs carrying white factors, and similarly the cocks
are producing the same two kinds of spermatozoa. We
have thus the following possible combinations in syngamy :

Eggs. Spermatozoa. Zygotes.

Black x Black Black.

White x Black Gray.

Black x White Gray.

White x White White.

As the combination of the different classes of gametes
is supposed to take place by chance, we will get on an
average of a large number about equal numbers of each
combination — i.e. the zygotes will be in the proportions
of one black, two gray, one white.

It must be understood that these numbers must not
be expected to be realised exactly. The position can be
paralleled by putting an equal number of white and black
counters into a box, shaking them up, and then drawing
them out in pairs without looking at them. Theory will
then lead us to expect that 25 per cent, of the pairs will
consist of two whites, 25 per cent of two blacks, and the
remaining 50 per cent, of one white and one black. It
will of course be only by chance that these percentages
are exactly realised, just as if we tossed up a penny a
hundred times we should probably get an approximately
equal number of heads and tails, but only by chance
exactly fifty of each. Thus in one experiment with the
Andalusian fowls the offspring of a number of them
totalled up to

Black, 41 ; Gray, 78 ; \Yhite, 39.

The further breeding of these offspring of the gray
hybrids is easily understood. Since blacks lack the white
factor altogether, when bred together they can give

HEREDITY 97

nothing but blacks, and similarly the whites can give
nothing but whites ; while the grays, having the same
constitution as their gray parents, when bred together
give the same results.

It is not always the case, however, that the character-
istics of the parents produce an appearance of blending
in the offspring as in the case of the gray Andalusian fowl
bred from a black and a white parent. If for instance we
cross a red Antirrhinum (Snap-dragon) with a white one,
the hybrids are not pink, but as red as the red parent.
If we breed together two of these hybrid reds, or take
seed from such a plant fertilized by its own pollen, we
get red and white offspring in the proportions of three
reds to one white. Further breeding shows that these
whites bred together produce only white offspring, while
the reds are of two kinds. Out of every three reds there
is one which is incapable of giving any but red offspring,
and two which show themselves to be of the same com-
position as their hybrid red parent, giving mixed offspring
in the proportion of three reds to one white.

The fact that the hybrid red, though indistinguishable
superficially from the pure red, gives a proportion of
whites among its offspring, shows that the characteristic
of whiteness is present, though concealed. In such
cases the characteristic which is concealed is called
recessive, \vhile the stronger characteristic which alone
makes its appearance is known as dominant.

A glance at Fig. 48 will show the relation of this result
to the case of the Andalusian fowl. Supposing that
black were dominant over white instead of blending with
it, all the grays would appear as blacks, and the pedigree
would read as follows : Line 2 — hybrid black ; line 3-
three blacks (i.e. one pure black and two hybrid blacks),

s. H. i ;

SEX AND HEREDITY

%

one white ; line 4 — one of the blacks (the pure one) of
line 3 produces only blacks, the other two blacks each
produce three blacks (one pure and two hybrid) and one
white, as before. Whites, of course, produce nothing
but whites.

This is obviously the same as the case of the snap-
dragon, reading red for black. It will also be appreciated
that the way in which the contrasted characteristics of
the parents behave in the body of the hybrid, — blending
or otherwise — is a purely subsidiary point, which does
not in any way affect the important matter of their
separation in the hybrid's gametes.

Although a case of blending characteristics has been
chosen from the animal kingdom, and non-blending ones
from the vegetable kingdom, it must not be supposed
that these two types of inheritance are peculiar to animals
and plants respectively. On the contrary, experiments
on a very large number of characteristics — colours, sizes,
shapes, etc., have shown that both kinds of inheritance
are found in both kingdoms, animals and plants exhibiting
an extraordinary similarity in the phenomena of heredity,
which can only mean that these are dependent upon the
most fundamental properties of living matter.

On a first consideration it would appear, having regard
to our own experience of inheritance in man, that Mendel's
law has a limited application. This is not the case, however,
and it is conceivable that ultimately all inheritance will
be shown to be of this type, it being merely the conse-
quence of the -hereditary material being composed of
small units. Though here we are entering speculative
regions, where we are far from reaching general agreement
amongst biologists, it may be explained that it is supposed
that the colour of the feathers (for example) is dependent

HEREDITY 99

upon a ' factor ' residing in one of the chromosomes.
To give a mental picture we may suppose that it is one
of the little chromatin beads as shown in Fig. 46, H.
Each gamete, as we have already seen, has one set of
chromosomes (in the fowl the number is seventeen instead
of the three in the species of Cyclops figured), but the
zygote has two sets. Hence while each gamete has but
one chromatin bead or factor for feather colour, the
organism (zygote) has two. Hence its hybrid nature if
the two are dissimilar. The gametes which issue from
this hybrid individual have, however, again only one set
of chromosomes, and therefore only the one colour factor,
and therefore conform to Mendel's la\v.

The reason why Mendel's law is difficult or impossible
to detect in many cases is certainly that the characteristics
under consideration are complex ones, depending upon
the interaction of many such factors, which exist in pairs
in the zygote and in single in the gamete. The variety
of ways in which the double set of factors of the zygote can
be sorted out into the single set of the gamete, and the
still greater variety of combinations that can result in
syngamy of the various kinds of gametes will then result
in such an enormous variety of zygotes as to render
impossible the discovery of segregation, which is only
detectable when the different types can be divided into
a few well-marked categories.

LECTURE VI
HEREDITY IN MAN

MOST of our knowledge of inheritance in man has had
perforce to be acquired by methods different to those
used with the lower animals, or plants, owing partly to
the fact that the experimental method is inapplicable,
and partly to the fact that the characteristics in which
we are for practical reasons most interested in man, are
apparently not simple ones in the Mendelian sense — i.e.
depending for their expression on only one or few factors
in the germ-plasm, but complex ones depending upon
the interaction of many.

Some simple cases of Mendelian inheritance in man are
known, however, most of them concerning abnormalities.
The inheritance of the condition known as Brachydactyly,
or short fingers, is shown in the family history summarised
in Fig. 49, the pedigree reading from above downwards
as in the case of Fig. 48. Persons with this abnormality
possess only two joints to the fingers and toes instead of
three. All fingers and toes are affected alike, and the
condition is associated with shortness of stature.

The first point to notice is that no case is known of
two brachydactylous persons marrying each other. Hence
the wives or husbands of the persons appearing in the
pedigree are not shown, they being all normal.

100

HEREDITY IN MAN

101

—

f

*.

1
> 0 «

r c

? ^

3

<

?c c

? <

!

? 5

1, 1 1 II

1 '. ',. ',. '- '» '* ' '

, i . LI

i

1

i i i,i i

i i i i i .

990 o @

FIG. 49.

Pedigree of Brachydactylous family, slightly condensed from Drinkwater.
(Proc. Roy. Soc., Edinb.. 1908.)

^ Brachydactylous male. ? Brachydactylous female.

$ Normal male. $ Normal female.

Figures in circles represent so many normal individuals without discrimination
of sexes.

A glance at the table will show that brachydactylous
persons (mated, as we have just seen, with normals)
produce both normal and brachydactylous offspring,
while normal individuals (also mated with normals)
produce only normal offspring. We therefore conclude
that the normal members of the pedigree produce nothing
but normal gametes, while the brachydactylous persons
produce both normal and brachydactylous gametes. In
short, the normals are comparable to the white fowls,
and the brachydactylous persons to the gray ones in
Fig. 48. The main difference between the two pedigrees
consists in the fact that the brachydactylous individuals
instead of being mated with their like, are mated with

102 SEX AND HEREDITY

normals. Hence we have the following possibilities in
syngamy :

Gametes of Brachydactylous Gametes of Normal

Persons. Persons. Zygotes.

Brachydactylous x Normal Brachydactylous.

Normal x Normal Normal.

Thus the result of the marriage of brachydactylous and
normal persons should be, on the average, half the children
brachydactylous and half normal — a result which is
approximately realised in the pedigree.

However, for the reasons indicated above, Mendel's
law will not take us very far in the investigation of
inheritance in man, and different methods have to be
employed which, while they may not have added much
to our knowledge of the how and why of inheritance, have
resulted in the discovery of formulae which describe in
a simple way the relation between parent and offspring
for a large number of characteristics.

The invention and development of these methods is
almost entirely due to Sir Francis Galton and Professor
Karl Pearson. The methods are much too elaborate
and require far too advanced a knowledge of mathe-
matics to attempt to describe here, but the general way
of attacking the problem and the main results are
easily intelligible.

To begin with, we will take the inheritance of height
or stature, using the data provided by Pearson's measure-
ment of over 1000 families. In this case we will limit
ourselves to fathers and sons, as the results are practically
identical for mothers and daughters, and indeed for fathers
and daughters and for mothers and sons.

In Table I the left hand column gives the heights of
the fathers (in inches) and the third column the average
height of their sons. The other two columns show the

HEREDITY IN MAN

103

number of inches by which the fathers or sons in question
differ from the average height of all the fathers (67-5
inches) or of all the sons (68-5 inches) respectively. The
fact that the average height of all the sons is an inch
greater than that of all the fathers need not concern us
particularly, as it affects the whole series of sons equally,
and a discussion of the reason for the difference would
be beyond our present scope.

Let us consider as an example the fathers of 65-5 inches
tall (which class includes all between 64-5 and 66-5 inches),
or two inches less than the average ( - 2 in column 2) .
There were 237 of these fathers, with the same number
of sons.1 The heights of these sons varied from 60 to
74 inches, but the average of them all was 67 inches-
i.e. 1-5 inches below the average for the total series of
sons.

TABLE i. Inheritance of Stature. All figures are in
inches . Correlation = • 5 .

Fathers.

Sons.

Heights.

Difference from
Average.

Heights.

Difference from
Average.

61-5

-6

66

-2-5

63-5

-4

66-5 -2

65-5

— 2

67 -i-5

67-5

0

C8-5

0

69-5

+ 2

69-5

+ 1

7J-5

+ 4

70-5

+ 2

73'5

+ 6

7-

+ 3'5

Running our eye over the Table we see that it is in-
variably the case that fathers measuring so many inches

1 Owing to the method of arranging the data, both in this Table
and in Table 2, the number of fathers is always equal to the
number of sons.

104 SEX AND HEREDITY

above or below the average have sons whose average
heights differ from the average of all the sons in the same
direction (above or below) as their fathers, but to a
smaller degree. And the exact working out of Pearson's
figures show that the degree is about one half. In other
words, sons on an average inherit half the peculiarity of
their fathers.

A word in passing may be said about a point which may
occur as a difficulty to some readers. At first sight it
might appear that the fact that tall fathers have sons
on an average less tall than themselves, and that short
fathers similarly have sons on an average less short than
themselves implies that the population is getting more
mediocre and less variable in each generation. This,
however, is of course not the case, and the reason is seen
at once when it is remembered that we are only dealing
with average heights. As we saw above, the sons of
fathers 65-5 inches tall ranged from 5 feet to 6 feet 2 inches,
and in this way, although on the average sons are less
peculiar than their fathers, there are always a number
of them even more extreme, and thus the supply of tall
and short men in the population is kept up.

We thus see that we cannot predict the height of
individual sons of individual fathers, but we can say in
general that given a large number of fathers of a given
height the average height of their sons will be so and so.
And that average height is obtained thus. If the height
of the fathers is so many inches above or below the
average, the average height of their sons will be about half
that number of inches above or below the average. This 1

1 Readers versed in statistical mathematics will know that
properly speaking the prediction of the heights of sons from that
of fathers is not obtained directly from the coefficient of corre-

HEREDITY IN MAN 105

figure, one half or -5 is termed the coefficient of correlation
between father and son, and is in this case a measure of
the intensity of inheritance. The method of arriving at
the exact figure is much more elaborate than the above
condensed table would seem to imply, and is a general
method of measuring the dependence of one thing upon
another. It is a figure which, when the dependence is
complete, gives the figure i ; when there is complete
independence, the figure o ; and when the dependence is
partial, some figure in between. Thus, in the case in
question, if the tallest fathers always had the tallest
sons, the next tallest fathers the next tallest sons, and so
on, the correlation would be i--i.e. inheritance would be
complete. If, on the other hand, there were no inheritance,
and tall fathers just as often had short sons as tall ones,
then the correlation would be o.

It is quite essential to grasp clearly this idea of corre-
lation, the measure of dependence of one thing upon
another, as it is impossible to understand otherwise the
bulk of the work done upon inheritance in man.

With this knowledge of the inheritance of a typical
physical characteristic which can serve as a standard of
comparison, we can consider the inheritance of a variety
of different characteristics, and we will begin with
Insanity, a condition which as everyone knows is liable
to run in families. One of the best investigations on this
subject was that made by Heron, who used as a basis
the archives of the James Murray Royal Asylum, Perth,
which contains the records of 331 family trees. Heron

lation, but through the mediation of the regression coefficient. It
does not, however, appear necessary to burden the untechnical
reader with this refinement.

io6

SEX AND HEREDITY

divided his material into males and females, treating the
sexes separately. Table 2 gives the results for the male
pedigrees, i.e. fathers and sons. The female pedigrees
gave similar results.

TABLE 2. Inheritance of Insanity. Correlation = -6.

Numbers actually
found.

Numbers to be
expected if there
were no inheritance.

Father and son (

Both sane.

26,774

26,728

alike-

Both insane,

49

3

Father insane, son

Father and son

sane,

149

195

unlike—

Father sane, son

insane,

361

407

First, let us take the first number (both father and son
sane) under " numbers actually found." It is clear that no
lunatic asylum records will be able to furnish this figure,
since it is only families of which at least one member is
insane that come within the ken of such an institution.
Nevertheless it is of the utmost importance to discover
this figure if we wish to get a true measure of the inherit-
ance of insanity, and it can be calculated from the known
percentage of insane in the general population. The
number of certified lunatics in Scotland on ist January,
1901, was 15,475 or -6 per cent. This, however, only
gives those certified insane on the date in question, and
takes no account of those who have been insane at some
period of their lives and subsequently discharged as
' cured/' It is reckoned that the percentage of the
population which is or has been at some time or other
certified as insane is about 1-5 per cent. On this basis
it is easy to calculate the figure 26,774 to correspond on

HEREDITY IN MAN

107

a 1-5 per cent, basis to the number of insane sons dealt
with in the table.

The column ' numbers to be expected if there were
no inheritance ' shows the number to be expected if
insane fathers were no more likely than sane fathers to
have insane sons, and is calculated on the same 1-5 per
cent, basis. Thus there were 198 insane fathers, of whom
49 had insane offspring and 149 had sane ones. But if
there were no inheritance- — that is to sav, if it were a

^ 7

pure matter of chance as to whether insane fathers had
sane or insane sons, we should expect only 1-5 per cent,
of the 198 sons — i.e. only 3 — to be insane, instead of the
49 actually found.

We are now in a position to consider the Table witfr a
view to discovering whether insanity is inheritable or not.
If inheritance is at work we should expect to find an
excess of cases where father and son are alike, and a
deficiency of cases where they are unlike . and on examining
the Table we find that this is the case. By the application
of appropriate mathematical methods we can measure
the intensity of inheritance in the same way as we measured
it for stature, and hence compare it with this as a standard.
If this be done the correlation is found to be -6 — that is
to say the records of the Perth asylum disclose an intensity
of inheritance of insanity 20 per cent, stronger than the
inheritance of stature, though too much reliance must
not be placed upon the exact figure.

A large number of human characteristics can be
examined on the same principle. Let us take another
mental characteristic, namely Ability.

In this case a slight modification of the usual method
has to be employed. It is very difficult to estimate the
relative ability of adults. It is easy, however, to get

io8

SEX AND HEREDITY

some measure of intelligence in children. For instance,
the class attained in the same school at a given age is an
automatic indicator of a certain kind of intelligence.
Therefore in this investigation we measure the resemblance
between brothers or sisters, instead of between parents
and offspring.

Table 3 summarises the investigations of Schuster and
Elderton on the school records at Charterhouse. The
' division ' in which each boy was situated when he
reached the age of sixteen was extracted from these
records, and the Table compiled in the usual way. (The
divisions are numbered from .1 to 7, the first being the
highest.)

TABLE 3. Position of pairs of brothers in Charterhouse
School at age 16. Correlation = -46

Numbers to be

Numbers actually

expected if there

found.

were no resemblance

between brothers.

/"Both above 4th

Brothers

division,

638 508

alike-

Both in 4th divi-

sion or below,

504 374

Brothers unlike — one above 4th

division and the other in 4th

division or below.

610

870

Total,

1752

!75^

It is plain that we can measure the intensity of re-
semblance between pairs of brothers, etc., in the same
way as we measured it between parent and offspring,
and arrive at a figure for the correlation. In the above
Table this figure works out to -46. Now in the case of

HEREDITY IN MAN 109

stature, where correlation both between father and son
and between brothers has been measured, it is found that
they give very nearly the same figure. If, therefore, we
find that in the above Table the correlation between
brothers is -46, we may take it that for all practical
purposes this is also the intensity of inheritance.

A similar investigation for Harrow School gave a
slightly lower value, the average for the two schools being
about -4. We can, therefore, say that these investigations
indicate a strong inheritance of ability, though about
20 per cent, less intense than the inheritance of stature.
It must be mentioned, however, that Pearson's school
schedules, compiled on a different principle and with
different material, gave the intensity of inheritance of
intelligence as -5, or equal to that of stature.

Pearson has made an attempt to measure the inheri-
tance of the most varied human characteristics by the
same method of measuring the resemblance between
members of the same family. His method was to send
out schedules to a number of schools (nearly 200), con-
taining directions, and tables to be filled up for (i) pairs
of brothers, (2) pairs of sisters, (3) pairs consisting of
one brother and one sister, for a number of physical and
mental characteristics. Limiting ourselves again to the
results for brothers (those for the other combinations
being closely similar) we can take as examples Tables
4 and 5, noting again the excess of ' numbers actually
found ' over those ' expected ' in the cases where the
brothers are alike, and the deficiency in the cases where
they are unlike, thus indicating the action of in-
heritance.

no

SEX AND HEREDITY

TABLE 4. Assertiveness. Correlation = -5.

Brothers f Both shy, -

alike — ^Both self-assertive,

Brothers unlike — one shy, the other
self-assertive,

Totals,

Numbers to be
Numbers actually exPected if there

found.
679

399
494

were no

resemblance

between brothers.

545
265

762

1572

TABLED. Popularity. Correlation = -5.

Brothers ( Both popular,
alike—- [Both unpopular,

Brothers unlike — one popular, the
other unpopular,

Totals.

Numbers actually
found.

Numbers to be
expected if there

were no

resemblance

between brothers.

IIO

37°

1028

68

528

1624

1624

Table 6 gives some examples of the results of Pearson's
investigations, and we note the close similarity between
the intensity of resemblance (i.e. of inheritance) for
physical and mental characteristics, as calculated from
these school data. The objection might otherwise have
been raised that the figures for the mental characters
were of less value than those for the physical, owing to the
greater liability of the former to be influenced by (i)
errors of judgment, or of bias, on the part of the teachers
who filled up the schedules, and (2) the similarity of

HEREDITY IN MAN in

environment (home training, etc.), to which brothers are
subjected, and which might produce a resemblance in-
dependently of inheritance. Neither of these disturbing
factors can, however, operate to any appreciable degree
in the case of the physical characteristics considered,
which are taken from the same schedules and filled up
by the same teachers, so that the similarity in results
strongly supports the view that the resemblance between
members of the same family in regard to mental character-
istics is actually due to inheritance.

TABLE 6. Examples of the intensity of resemblance
between brothers for various characteristics.

Characteristic.

Correlation.

A. PHYSICAL.

Colour of eyes (light, medium, dark), -5

Hair (smooth, wavy or curly),

Breadth of Head,

B. MENTAL.

Vivacity (quiet or noisy),
Assertiveness (shy or self-assertive),
Popularity (popular or unpopular),
Conscientiousness (keen or dull),
Intelligence (six categories),

'5
•6

•5
•5

•5
•6

•5

The figures which we have discussed in this chapter
form only a small selection from the mass of data dealing
with a wide range of characteristics, mental and physical,
including the power of resistance to disease, which has
been collected by Pearson and his colleagues. \Yhile
they do not help us much to understand the physiology
of inheritance, in the way for instance that Mendel's

H2 SEX AND HEREDITY

discovery has done, and which should be the chief aim
of all scientific investigations of heredity, they possess
the great practical importance of emphasising the dangers
and possibilities which beset the human race.

Infirmity and disease are so common among civilised
man that we have come to look upon them as part of the
order of nature. Under conditions more natural in
regard to natural selection, however, these afflictions
would not have the chronic influence in our lives that they
have at present. When a wild animal falls ill it generally
either makes a quick recovery or dies. Thus the deformed
and unfit, and those which are less resistant to disease,
die off more rapidly, and therefore leave fewer offspring
to inherit their weUk constitution and poor powers of
resistance than do the more fit animals.

In this way the power of resisting disease is maintained
and increased under a state of nature ; but in the case of
man, and especially civilised man, humane considerations
coupled with medical science keep alive large numbers
of those who under natural conditions would perish,
and thus the evolution against disease is impeded or even
altogether suspended.

Now it cannot be denied that this is a very grave state
of affairs. Indeed it is doubtful whether any other
factor will affect more the welfare of the human race in
the future, and it is considerations such as these which
lead Eugenists to advocate the policy of a partial control
of the birth-rate, preventing the multiplication of the
obviously unfit (for example, the insane or feeble-minded)
and encouraging a high birth-rate among the vigorous
and intelligent.

The fact that the birth-rate in this country (as in all
the highly-civilised ones) is diminishing has been so

HEREDITY IN MAN 113

thoroughly ventilated in the popular press that everyone
is familiar with it. The really serious aspect of the
question, however, which has not been so well appreciated,
is the fact that the diminution has not affected all classes
of the community alike, so that very different birth-
rates obtain in the different classes of society. The
birth-rates for a number of different occupations are given
in the official publications of the 1911 Census of England
and Wales. A few representative samples, taken from
the two ends of the scale, are given in Table 7.

TABLE 7.

Birth-Rate per 1000
Occupation. married men under 55

years of age.

Teaching Profession, 95

Lawyers, TOO

Doctors, 103

Agricultural Labourers, -I 161

Dock Labourers, -
Coal Miners,

231
232

Now a differential birth-rate among the various sections
of the community means, unless there are counter-
balancing factors, that the sections with the lower birth-
rate will appear in smaller and smaller proportions in
succeeding generations. In other words, there is danger
of the average mental powers of the race declining, owing
to the failure of the more intellectual members to reproduce
themselves in proper proportion to the rest of the com-
munity.

It will be noted that the above conclusion assumes that
the members of the intellectual professions have on the
average greater mental capacity than those in the unskilled

S.H. H

ii4 SEX AND HEREDITY

trades, and that the difference is not merely due to
different education imposed on similar material. A proper
discussion of this question would take us far beyond the
scope of these lectures, and it must suffice to mention the
single point, that the fact that entrance into a profession
is dependent upon the ability to pass intellectual tests,
and that success therein depends mainly on intellectual
capacity, excludes all markedly unintelligent persons
from such a profession. The exclusion of these obviously
raises the average intellectual capacity of the members
of the profession above that of these engaged in occu-
pations into which men of all degrees of mental capacity
-high or low — are admitted.

This question, however, like the problem of the physi-
cally unfit members of the population, is capable of an
amount of argument quite out of place in a book of this
size and character, and here we must leave it, content with
having indicated certain practical problems arising out
of our study of heredity, problems which are vastly more
vital to the real welfare of the human race than those
political questions which usually absorb the energies of
governments.

GLOSSARY AND INDEX

Numerals in heavy type signify that an illustration is given on the

page quoted

Acctabularia, isogarnetes of, 22

aconite, dependence on Humble
Bee, 40.

acquired characters, those im-
pressed on the organism in the
course of its individual life : 65,
91.

adiantum, a Fern, embryo of, 36.

Agrostemma, pollen-tubes of, 43.

allantois 75, 76.

Alytes, 69; 70.

amnion, 75.

amphibians, reproductive adapta-
tions, 68.

Animal agency, in pollination, 39.

Antheridium, the male organ, for
instance of a Fern, which con-
tains spermatozoids, 25.

Aquatic stages, in development,
67

Archegonium, the female organ,
for instance of a Fern, which
contains one ovum, 26.

Balance, of human body, 52.

Birds, egg, 72, 74, 75 : embryo, 76

Birth-rate, 112.

Bladder-wrack (Fucus), 15 : pro-
pagation of, 1 8 : gametes and
fertilization of, 20.

Body-plasm, the protoplasm of the
body-cells, as opposed to the
germ-plasm 88.

Brown Seaweeds, sexual differ-
entiation in, 15-20.

Budding, in Plants ; a separation
of a part of the plant-body,
which grows directly into a
new individual, 4 : repeats the
characters of the parent, 7.

Carpels, the leaves at the centre
of a flower, enclosing the ovules,
28, 48.

Cell, the structural unit of the body
of animals and plants, 2, 3.

Cell-division, the ordinary method
of increase in number of cells,
one cell dividing into two, 3.

Cell-theory, 2.

Chromatin, the most conspicuous
constituent of the nucleus, and
generally identified as the phy-
sical basis of heredity, 85.

Chromosomes, bodies into which
the chromatin is concentrated
before division of the nucleus.
At other times the chromo-
somes are diffused through the
nucleus, and their individual
boundaries are not distinguish-
able, 83, 85.

Conjugation, the coalescence of
two equal gametes, 16, 22.

Co-operation, in living body, 53.

Copromonas, 54 ; life-history, 56.

n6

SEX AND HEREDITY

Correlation, coefficient of, 105 :
the measure of degree of depen-
dence of one variable upon
another.

Crossing, 38.

Cutleria, 18 ; 19.

Datura, seed of, 49.
Death, not a necessary sequel to
life, 60, 88 : in higher animals,

63-
Depression, 60.

Dominant, a character of one
parent, which develops in the
body of a hybrid to the ex-
clusion of the corresponding
character of the other parent, 97.

Echidna, 77.

Ectocarpus secundus, male and
female gametes of, 17.

Ectocarpus siliculosus, isogametes
of, 15, 16, 17.

Egg, 4 : of birds, 72, 74.

Embryo, internal in land-plants,
50 : of bird, 76 : of mammal,
poisoning or infection, 80.

Embryo-sac, a large cell within
which is the ovum in flowering
plants, 44.

Eugenics, the science of the im-
provement of the race, especially
through the agency of selection,
112.

Euglena, 9 : life history of, 10, 32.

Factor, (hereditary) : the hypo-
thetical physical entity in the
germ-plasm which is concerned
with the production of any
particular feature of the body ;

^ 95, 99-

Female gamete, or macrogamete,
or ovum : the female sexual
cell, which is usually non-
motile, and is larger than the
male gamete which fuses with
it': 4-

Fern, sexual propagation of, 24 :
attraction of spermatozoids by
ovum, 34.

Fertilization, the fusion of the
male gamete or spermatozoid
with the female gamete or
ovum : the term is equivalent
of syngamy : in a Fern, 26 :
in a flowering plant, 29, 37, 41,
46 : in Cyclops, 82, 83.

Fish-like characters in embryos/
67.

Fission, in Euglena, 10 : in Copro-
monas, 55 : in body of higher
animals, 62.

Flower, a dense group of organs
borne on the end of a stalk in
flowering plants, in connection
with which the gametes are
produced, 27.

Foot, the suctorial organ of the
embryo in Ferns, 35, 36.

Frog-spawn, 68.

Fitcus, gametes of; 3 : fertiliza-
tion of, 20 : attraction of
spermatozoids by ovum of, 33.

Funiculus, stalk of ovule or seed,
42, 49.

Fusion of sexual cells, or syngamy,

3-

Gametangium, a cell which pro-
duces gametes : of Ectocarpus,
17 : of Cutleria, 19.

Gamete, or sexual cell : a cell
which is capable of fusing with
another cell in a sexual act, 3 :
of Fucus, 5 : of man, 5 : of
Ulothrix, 12 : of Ectocarpus, 16,
17 : of Cutleria, 19 : of Orchis,
29 : of Copromonas, 57 : of
Plasmodium, 59 : of Stylo-
rhynchus, 58.

Germ-cell, a gamete, or cell which
will give rise to a gamete, 81.

Germ-plasm, living substance
which is able to give rise to
complete new individuals. 88.

GLOSSARY AND INDEX

ii

Gill-openings, in young bird, 76.

Gonad, the aggregate of the germ-
cells, 64.

Grass, wind-pollination of, 38.

Green algae, sexual differentiation
in, 21.

Heredity, 6.

Hermaphrodite, of flowers, those

which contain both stamens

and carpels, 37.
Humble-bee, agent of pollination

of aconite, 40.
Hyla, ~o.

Immobility of plants, its effect
on propagation, 38.

Immortality, potential, Protozoa,
6 1 : germ-cells, 64 : germ-
plasm, 88.

Impressed characters, not in-
herited, 65, 91.

Inheritance, absence in impressed
characters, 65, 91 : colour in
fowls, 94 : colour in snap-
dragons, 97 : brachydactyly,
100 : stature, 102 : insanity,
105 : ability, 108 : mental
characteristics, in : physical
characteristics, in.

Isogametes, those which are of
equal size and structure, 4.

Land, adaptation for life on, 66.
Life, nature unknown, 51 : com-
plexity of vital processes, 51.
Lily, ovary of, 48.

Macrogamete, the female gamete
or ovum, which is usually non-
motile, and is larger than the
male gamete which fuses with
it : 4, 82.

Male gamete, or microgamete, or
spermatozoon, or spermatozoid,
the male sexual cell which is
usually motile, and is smaller

than the macrogamete, with
which it fuses : 4.

Malic acid, its effect on movement
of the spermatozoids of Ferns, 35.

Mammals, 76.

Man, gametes of, 5.

Marsh Marigold (Caltha), ovule
of, 44 : carpels of, 48.

Medium, internal, 67.

Mendel's Law, 94, 98, 100.

Microgamete, or spermatozoon,
or spermatozoid, the male ga-
mete, which is smaller than the
ovum with which it fuses : 4.

Micropyle, the narrow channel
through which the pollen-tube
passes to the ovum in flowering
plants : 44, 45.

Mortality, of animal body, 63.

Motility of gametes, 22 : of
animals and plants, 32 : of
spermatozoids in water, 33.

Multicellular organisms, which con-
sist of a number of cells, 2.

Narcissus, pollen-tubes of, 43.

Nephrodium, a Fern : antheridia
and spermatozoids of, 24.

Nototrema, 71, 72.

Nucleus, a definite, frequently
rounded body within the proto-
plasm, which contains chro-
matin and is the controlling
centre of the cell : 2, 3.

Onoclea, a Fern : fertilization of,
26.

Organization of animal body, 62.

Ornithorhynchiis, 77.

Ovule, the future seed of flowering
plants, which at the period of
flowering contains an ovum,
28, 42, 44, 48.

Ovum, the female gamete, which
is usually non-motile, and being
larger than the male gamete
which fuses with it, it is often
called the macrogamete, 4, 5.

nS

SEX AND HEREDITY

Phyllobates, 70.
Phyllomedusa, 68.
Pipa, 71, 73.
Placenta, 79.
Plasmodium, 58, 59.
Pollen-grain, 45.

Pollen-tube, the result of germi-
nation of the pollen-grain, 28 :

41- 42.

Pollination, the transfer of pollen
from the anther to the stigma
of the flower, 37.

Polygonum, ovary of, 42.

Polvpodium, a Fern, archegonia
of, 25.

Prothallus, the sexual stage of a
Fern, which bears the sexual
organs, 25.

Protococcus, 11, 32.

Protoplasm, the physical basis of
life, 2 : necessarily moist during
life, 66.

Protoplast, the whole protoplas-
mic body of the cell.

Protozoa, compared with higher
animals, 62.

Quince, flower of, 28.

Rabbit, transference of eggs of, 79.

Recessive, a character of one
parent, which is masked in the
body of the hybrid by the ex-
clusive development of the
corresponding character of the
other parent, 97.

Reproduction, complexity of, 53.

Rhacophorus, 69.

Rhinoderma, 71.

Salvia, pollination-mechanism of,
40.

Segregation (Mendelian), the dis-
sociation of the parental char-
acters— or rather of their
factors — during the formation
of the gametes, 95.

Sexual cells, or gametes, those
cells which take part in syn-
gamy, 3 : minute and micro-
scopic, 8 : of Fucus, 5 : of Man,
5.

Sexuality, 2.

Shepherd's Purse, seed of, 49-

Soma, the substance of the
animal body apart from the
gonad, 64.

Specialisation of cells, 63.

Spermatozoid, the microgamete of
plants, 4 : of Fucus, 20 : of
Fern, 24 : of Zamia, 47.

Spermatozoon, the microgamete
of the higher animals, 82.

Spontaneous generation, unknown
to scientific men, 6.

Stamens, the parts of the flower
which produce the pollen, and
ultimately the male gametes, 27.

Stigma, the receptive surface of
the carpel, 28.

Structural unit, the cell of animals
and plants, 2.

Stylorhynchus, 57, 58.

Surinam Toad, 71, 73.

Syngamy, the fusion of two
sexual cells, and especially of
their nuclei, to form one zygote,
3 : in Fucus, 20 : in Fern, 26, 27 :
in flowering plants, 30 : in
Copromonas, 57 : in Plasmo-
dium, 59 : in Stylorhynchus, 58 :
in Cyclops, 82 : necessity of,
60 : effect in staving off death,
61.

Tissue, a mass of cells of common
origin, and showing a common
life, composing the body of an
animal or plant, i.

Ulothrix, 12 : ^egetative increase
of, 13 : gametes of, 14, 32.

Unicellular organisms, which con-
sist of only one cell, 2.

GLOSSARY AND INDEX

119

Vaucheria, gametes of, 22.

Vegetative increase of number by
separation of a part (bud), or
a single cell from the parent,
without syngamy : in Ulo-
thrix, 13.

Yolk, 72.

Zamia, spermatozoids of, 47.

Zoo-spores, or swarm-spores, naked
cells capable of movement in
water, which reproduce the
plant vegetatively : of Ulo-
thrix, 13.

Zygote, the result of fusion of
sexual cells or gametes : it is
the starting point of a new in-
dividual, 3 : of Ectocarpus, 16 :
of Copromonas, 57 : resting
stage, 6 1 : of Stylorhynchus, 58.

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