Let us now turn back to some of the earlier pages of the embryological record which we passed over in order that we might translate the later portions dealing with more familiar and intelligible structures like gills. Before the egg of the frog becomes an elliptical mass of cells, it is at one time a double-walled sac enclosing a central cavity; in this stage it is called a _gastrula_. Tracing back the mode of its formation, we find that it is produced from a hollow sphere of fewer cells that are essentially alike; this stage also is so important that the special term _blastula_ is applied to it. Still earlier, there are fewer cells--128 or thereabouts, 64, 32, 16, 8, 4, 2, and 1. In other words, the starting point in the development of the frog is a _single biological unit_; this divides and its products redivide to constitute the many-celled blastula and the double-walled gastrula. All the other animals we have mentioned begin like the frog, as eggs which are single cells and nothing more; they too pass on to become blastulae and gastrulae, similar to those of the frog in all essential respects, particularly as regards the nature of the organs produced by each of the two primary layers, and the mode of their formation. Does the occurrence of blastulae and gastrulae and one-celled beginnings mean that the higher animals composed of numerous and much differentiated cells have evolved in company from two-layered saccular ancestors which were themselves the descendants of spherical colonies of like cells, and ultimately of one-celled animals?