Accordingly, if we advance from that primitive animal type, the volvox, consisting of a simple group of cells arranged spherically, like an elastic rubber ball, to the cœlenterata, we meet with the phenomenon of the first invagination, producing an animal body consisting of two layers of cells and an internal cavity, communicating with the exterior by means of a pore or mouth. The two layers of cells promptly divide their task, the outer layer becoming protective and the inner nutritive; and in consequence of their different functions, the cells themselves alter, the outer layer acquiring a tougher consistency, while the inner remains soft in order to absorb whatever nutriment is brought by the water as it passes through the mouth. In this way, there is a division of labor, such that all the external cells protect not only themselves, but the whole organism; while the internal cells absorb nutriment not only for themselves but for the others. This is the simplest example of a process that becomes more and more complex in the formation of higher organisms; in adapting themselves to their work, the cells become greatly modified (formation of tissues) and perform services that are useful to the entire organism. And at the same time, because of the very fact that they have been differentiated, they become dependent upon the labors of others, for obtaining the means of subsistence. Similar laws seem to persist even at the present day in the formation of social organisms, in human society.
During the development of the embryo, all animals pass through similar phases; and to this man is no exception.
Fig. 1.—Human Ovum, Magnified. a. Vitelline membrane; b. Vitellus; c. Germinal Vesicle.
He traces his origin to an ovum-cell formed of protoplasm, nucleus and membrane, measuring only a tenth of a millimetre, yet vastly large in comparison with the spermatic cell destined to fertilise it by passing through one of the innumerable pores that render the dense membrane penetrable.
Fig. 2.—First Segmentation of a Fertilised Ovum.
Fig. 3.—A Morula as seen from the Outside.
