(oogonia), divide and redivide, often forming balls of cells. They give rise to spermatocytes, which also multiply by division. The last chapter in the spermatogenesis is the transformation of spermatocytes into spermatozoa.

In some cases spermatozoa which have been transferred to a female may lie long dormant there. Thus those received by the queen-bee during her nuptial flight may last for a whole season, or even for three seasons, during which they are used in fertilising those ova which develop into workers or queen-bees.

7. Maturation of the Ovum.-Before an egg-cell is fertilised it usually exhibits a remarkable process of maturation. The nucleus moves to the surface and divides twice in rapid succession, forming two minute cells or polar bodies, which are extruded and come to nothing. The first of these two divisions is usually quite unique, for by it the number of chromosomes is reduced to half the normal number, whereas in ordinary nuclear division each chromosome is cleft longitudinally and the number remains the same. In the formation of the spermatozoa there is a similar reducing or meiotic division, and thus when the spermatozoon and ovum unite in fertilisation, the normal number of chromosomes is restored. The reduction of the chromosomes by a half is probably of great importance in connection with variation and heredity. It enables us to understand how certain items in the inheritance may drop out altogether, or how they may be represented only in a certain proportion of the offspring.

8. Fertilisation.-When a pollen grain is carried by an insect or by the wind to the stigma of a flower, it grows down through the tissue of the pistil until it reaches the ovule and the egg-cell which that contains. Then a nuclear element belonging to the pollen cell unites with the nucleus of the egg-cell. The union is intimate and complete.

When spermatozoa come in contact with the egg-shell of a cockroach ovum, they move round and round it in varying orbits until one finds entrance through a minute

aperture in the shell. It works its way inwards until its nuclear part unites with that of the ovum. The union is again intimate and complete.

The result of fertilisation is the intimate and orderly union of the nucleus of the egg-cell and the nucleus of the sperm-cell, but it must be remembered that there is also a mingling of the infinitesimally minute cell-substance or cytoplasm of the spermatozoon with the rela▪ tively large cell-substance of the ovum. Some idea both of the orderly complexity of the nuclear union and of the carefulness of modern investigation may be gained from the fact that the nucleus of each of the two daughtercells which result from the first division of the fertilised egg-cell is made up of chromatin contributions half maternal and half paternal. This equal partition has been followed for a number of successive divisions, the actual demonstration of this being rendered possible in some cases by a visible difference between the maternal and paternal chromosomes.

Five chief things happen when an animal ovum is fertilised by a sperm. (1) There is a mingling of two inheritances, of the maternal and paternal germ-plasms. (2) There is a restoration of the number of chromosomes to the normal. As the ripe ovum has had its number reduced to one half of the normal, and as the same is true of the ripe spermatozoon, the union of the two must bring back the normal number, which is usually adhered to in all the cells of the offspring. (3) The spermatozoon brings into the ovum a minute body known as the centrosome, which plays an important part in the subsequent division or segmentation. (4) When spermatozoon enters the ovum, there is a very rapid physical change in the periphery. The ovum becomes nonreceptive to other spermatozoa. The way is "blocked," and this usually prevents multiple fertilisation, which is one of the causes of abnormal development. (5) There is some stimulus to the egg to divide or the removal of some embargo which has kept the egg from dividing. A very remarkable fact, established by Loeb, Delage, and others, is that this part of fertilisation-the setting

a

of the egg-cell a-dividing-can be induced in many cases by a great variety of stimuli. Such a development, without the aid of a spermatozoon-is described as "artificial parthenogenesis. The stimuli may be mechanical, chemical, thermal, electrical, and they are known to work effectively in a great variety of cases,some starfishes, sea-urchins, worms, insects, molluscs, fishes, and amphibians. It may serve to leave seaurchin ova for a short time in sea-water with slightly altered composition and concentration, or to prick frogs' eggs with a platinum needle and wash them in blood. If development is to proceed normally-and both seaurchins and frogs have been reared from artificially parthenogenetic ova-two steps seem to be necessary. The egg-cell must be activated (by a positive stimulus or by the removal of some obstacle to division) and there must be an immediate corrective or counteractive of this, otherwise the segmentation of the egg will simply end in disintegration.

CHAPTER XV

DEVELOPMENT

1. Segmentation and after-2. Differentiation-3. Some generalisations: (a) the ovum theory; (b) the gastræa theory; (c) recapitulation; (d) organic continuity.

1. Segmentation and After. The fertilised egg-cell divides, and by repeated division and growth of cells every embryo, of herb and tree, of bird and beast, is formed. On the quantity and arrangement of the yolk the character of the segmentation in part depends, but there are other factors involved. When there is little or no yolk the whole ovum divides into equal parts, as in sponge, earthworm, starfish, lancelet, and higher mammal. When there is more than a little yolk, and when this sinks to the lower part of the egg-cell, the division is complete but unequal, and this may be readily seen by examining freshly laid frog-spawn. When the yolk is accumulated in the core of the egg-cell, the more vital superficial part divides, as in insects and many crustaceans. Lastly, when the yolk is present in large quantity as in the ova of gristly fishes, reptiles, and birds, the division is very partial, being confined to a small but rapidly extending area of formative living matter, which lies like a drop on the surface of the yolk.

As the result of continued division, a ball of cells is formed. This may be hollow (a blastosphere), or solid (a morula, i.e. like a mulberry), or it may be much modified in form by the presence of a large quantity of yolk. Thus in the hen's egg what is first formed is a disc of cells technically called the blastoderm, which gradually spreads around the yolk.

The hollow ball of cells almost always becomes dimpled in or invaginated, as an india-rubber ball with a hole in it might be pressed into a cup-like form. The dimpling is the result of inequalities of growth. The two-layered sac of cells which results is called a gastrula, and the cavity of this sac becomes in the adult organism the digestive part

(a)

(2)

FIG. 103.-THE FORMATION OF THE Two-
LAYERED GASTRULA FROM THE IN-
VAGINATION OF A HOLLOW SPHERE
OF CELLS.

(From the Evolution of Sex; after
Haeckel.)

of the food - canal. Where there is no hollow ball of cells, but some other result of segmentation, the formation of a gastrula is not so obvious. Yet in most cases some analogous infolding is demonstrable.

In the hollow sac of
cells there are already
two layers. The outer,
which is called the ecto-
derm or epiblast, forms
in the adult the outer
skin, the nervous sys-
tem, and the most im-
portant parts of the
sense-organs.
The in-

ner, which is called the
endoderm or hypoblast,
forms the lining of the
most important part of
the food-canal, and of
such appendages
lungs, liver, and pan-

as

creas which are outgrowths from it. But in all animals above the Sponges and Coelenterates, a middle layer appears between the other two. From this-the mesoderm or mesoblast-the muscles, the internal skeleton, the connective-tissue, etc., are formed.

2. Differentiation.-Development is the expression or realisation of the inheritance. It is the making visible of