Like the protozoon, the ovum, immediately after its impregnation, begins to undergo a certain division, by a series of successive segmentations, into 2, then 4, 8, 16, 32, 64, etc., cells. By continuous cell segmentations a great mass, the morula or mulberry, is produced. The structure of the morula corresponds with that of the coelenterata, or the animals of the second type. To this type belong the animals with gemmiparous reproduction, or multiplication by means of buds. The divided animals remain together and form colonies, as e. g. in sponges or corals. The next event in the formation of the embryo is the blastula. The solid spherical mass of cells becomes hollow like a rubber ball. In the subsequent stage the blastula becomes flat at one pole. By degrees a depression is formed at this point, which becomes deeper step by step, until the inner layer reaches the outer layer, representing half a sphere of two layers, like a collapsed rubber ball. In the farther growth the edges approach the middle line till they finally meet and fuse together. The oval body, called the gastrula, thus consists of two layers, the primitive germinal membranes, the ectoderm and entoderm. The gastrula resembles in its structure that of the worms, or the third type of the animal kingdom. [Illustration: CUT II. _Diagram, showing the development of the organs._ ek, ektoderm; mp, medulary plate; ms, mesoderm just forming from the ed, entoderm; c, coelom or body cavity; nt, nerve-tube or spinal cord; ac, abdominal cavity; cd, chorda dorsalis; it, intestinal cavity.] By certain foldings of the ento- and ectoderm transformations arise, and new organs develop. Two folds of the entoderm grow higher, approach each other and finally meet. In this way the embryo consists of four germinal membranes. A certain folding of the ectoderm marks the position of the future backbone in the primitive stripe. A longitudinal furrow marks the origin of the nerve-tube. The different membranes have thus formed several tubes, the chorda dorsalis, the definitive intestinal canal and the abdominal cavity or coelom. The structure of the embryo resembles now more or less that of the animals of the fourth type or the echinodermata. The membranes which include the intestinal canal soon overgrow on both sides the nerve-tube and the chorda and are then differentiated partly into the bones of the skeleton and partly into the muscles. In the meantime, the vascular spaces develop. At one point of the vascular tube a rhythmical pulsation is observed, representing the primitive heart, similar to that of the mollusca.