The science of life : fully illustrated in tone and line and including many diagrams

WEARING OUT OF MACHINE AND ITS REPRODUCTION

tissue that lines it is broken up and scrapped, and fresh tissue is grown instead ; thus there is always fresh, young tissue in the uterus, ready for the reception of the embryo. This overhaul does not coincide with the liberation of the ovum, which takes place about ten days or a fortnight after it has begun.

The ovum may encounter a spermatozoon at any time after it has started on its journey. Spermatozoa may or may not have entered the body to meet it. These minute invaders, where they are present, propel themselves by screw-like lashings of their tails, and receive some assistance in the earliest stages of their journey from muscular coritractions of the uterus itself. For an object as minute as a spermatozoon this journey of several inches is a tremendous undertaking. Only a small proportion of the two hundred million launched by the male reach the upper end of the oviduct. The majority perish on the way. But when the survivors reach that destination, their troubles are over ; they rest in the oviduct and retain their capacity for fertilization for at least a week.

It is probable that the ovum can live for three or four days in the oviduct after its escape from the ovary. If during this time it meets an active spermatozoon, it is fertilized and can fulfil its destiny. If not, it dies and disintegrates in

the uterus and is presently washed Fig. 62.

away. We will assume that the former alternative occurs, and we will trace the gradual development of the minute oyum into a young mammal.

The actual fertilization of the ovum is accomplished by one only of the myriads of searching spermatozoa. ‘The successful competitor presses its head against the side of the relatively colossal ovum, and as a result of that simple contact the latter is activated ; it ceases to be sterile, and begins forthwith the series of changes that constitute development. After the spermatozoon has activated the ovum it does something further. Its head burrows actually into the substance of the ovum, leaving the tail outside to wriggle for a while and then to perish. When it gets inside, the head swells up and becomes a typical nucleus which approaches and fuses with the nucleus of the ovum. ‘The resulting cell, having the cytoplasm of the ovum and a

compound nucleus derived partly from the original ovum and partly from the sperm, begins to grow and divide. It is important to realize that this fusion of nuclei has nothing to do with the activation of the ovum; it is the mechanism by which characters inherited from the male parent are transmitted to the offspring. Activation, the change that enables the ovum to develop, occurs at the moment when the sperm touches the ovum. It isa sort of microscopic laying on of hands that awakens the slumbering possibilities of reproduction.

Segmentation—the first steps in the development of a human egg (magnified about 150 diameters).

8 3

The Growth and Development of the Embryo

During its passage down the oviduct into the uterus—a passage that takes about a week—the fertilized ovum develops rapidly. It is, to begin with, a giant as cells go, and the first thing it does is to split itself up into a large number of cells of reasonable size. This process is called segmentation. It occurs by a series of divisions of cells into two; the original ovum divides into halves, then each half divides again, giving four quarters, then each quarter divides again into two eighths and so on.

By the time it reaches the uterus the ovum has turned itself into a mass containing some hundreds of cells. But it has done something else as well, for besides merely dividing and

93