From the fertilized ovum at the top there is a continuous cell development, with differentiation represented in the cell groups of the bottom row.

It is seen that the sexual cells are formed directly from the germ cell and contain no admixture from the cells of the body.] The mother gives the protection of a narrow and unchanging environment and food to the new organism which develops within the uterus, and there is always a membranous separation between them.

Disease of the mother may affect the foetus in a number of ways.

In most cases the membrane of separation is an efficient guard preventing pathogenic organisms reaching the foetus from the mother.

In certain cases, however, the guard can be passed.