The head and neck of the spermatozoon, which contain the nucleus and centrosomes, are the essential parts, and the middle piece and the remainder of the tail appear to be used solely for locomotion and penetration. When the head penetrates the ovum, the tail is detached and rejected. Our knowledge of the initial stages in the development of a human embryo is derived indirectly from the observation of other mammals. There are nine early human embryos reported, and the average probable age of these is twelve days. Breuss' specimen was probably ten days old (_Wiener med. Wochenblatt,_ 1877). Peters (_Einbettung des mensch. Eies,_ 1899) found a smaller embryo than this. The Breuss ovum was 5 mm. in length; Peters' was 3 by 1.5 by 1.5 mm., but the probable age was not given. There have been numerous embryos more than twelve days old observed, and since the process after the twelfth day is identical in man and the higher mammals, there is no doubt that the first stages are also the same. The segmentation that makes new cells is complicated, and the outcome of the division is a ball of cells. In eggs which have a large yolk, like those of birds, the cells form a round body resting on the surface of the yolk, but in mammalian ova a hollow ball of cells, or a _Morula,_ results, which lines the internal surface of the cellular envelope. The ovum absorbs moisture by osmosis and enlarges, and about the twelfth day after the germ-nuclei have begun to divide, the Morula, or hollow ball of cells, called also the _Blastodermic Vesicle,_ is formed. The next stage in development is the establishment of two primary germinal layers, called together the _Gastrula_, The outer layer is the _Ectoderm_ or the _Epiblast,_ and the inner layer is the _Endoderm_ or _Hypoblast_. In a Morula the smaller cells, which contain less yolk-material, gradually grow around the larger yolk-containing cells to form the Gastrula. Between the Ectoderm and the Endoderm a layer of cells called the _Mesoderm_ or _Mesoblast_ is next formed, and from these three layers all the parts of the embryo are built up. From the outer Ectoderm and the inner Endoderm those organs arise which are in the body, outer and inner,--as the nervous system and the outer skin from the Ectoderm, the inner entrails, the lungs and liver, from the Endoderm. From the Mesoderm come the inner skin, the bones and muscles.