=Segmentation of the Oösperm.= The fertilization of the ovum imparts to it a wonderful stimulus, so that the oösperm divides, by mitosis, into two cells, these two into four, the four into eight, the eight into sixteen, these into thirty-two, and so on repeatedly, until a large number of comparatively small cells are formed (Fig. 10). This mass of cells is spherical in shape, and the little round cells towards the surface project in such a way as to give to the mass an appearance somewhat similar to the fruit of the mulberry, whence it is termed the mulberry body or _morula_ (Fig. 10, 4). In the morula stage of his existence man resembles the solid colony of protozoans represented by _Pandorina_. The cells of the morula next become arranged regularly in a single layer at the circumference, by which the embryo assumes the form of a hollow sphere, and is known as the _blastula_ (Fig. 10, 5). This phase of man’s existence is quite suggestive of _Volvox_. Soon one side of the blastula is invaginated or pushed in, as one would push in one side of a hollow india-rubber ball. The result of this invagination (called _gastrulation_ technically) is the formation of a sort of cup. This is the _gastrula_ (Fig. 10, 6) phase of man’s existence. It is a higher and fundamentally different phase of existence than either the morula or the blastula. It corresponds, not to a Protozoan, but to the higher Metazoan. It possesses the fundamental anatomical qualities of a low Cœlenterate (Polyp). The mode of gastrulation is different in man from that just described, and varies in different animals, but the essential point common to all is the formation of a double cellular-membrane; the outer membrane being called _epiblast_ or ectoderm, and the inner one _hypoblast_ or endoderm, the enclosed cavity being the primitive digestive cavity. These two layers are the primary germinal layers. A third layer is subsequently formed between them, by the agency of one or both of them, and is called the _mesoblast_. From these three simple membranes, which are composed exclusively of cells, are formed all the complex tissues and organs of the adult man. The _epiblast_ develops into the nervous system (brain, spinal cord, and nerves), and the cuticle, hair, etc. The _hypoblast_ develops into the cellular parts of the digestive canal, the liver, lungs, etc. The _mesoblast_ develops into the muscles, bones, ligaments, blood-vessels, etc. To trace the details of this evolution of a human being from the microscopic _oösperm_ is a fascinating and instructive study, but is beyond the limits and purpose of this little book. It will well repay further careful study by the reader.