Problems of development concern chiefly (1) progressive differentiation, (2) orientation, coordination and regulation, (3) attainment of useful ends; the attempt is made to find causal explanations of each of these. The early differentiation of the egg is initiated by the escape of nuclear materials into the cytoplasm, the formation of specific substances there, the localization and isolation of these substances by protoplasmic movements and cell divisions. In Ascidian eggs inclusions such as yolk, pigment, mitochondria are not "organ-forming" and may be displaced by relatively weak centrifugal force without seriously modifying development, but if the hyaloplasm of different areas is displaced by strong centrifuging, larval organs are displaced. The polarity, symmetry and pattern of localization of this hyaloplasm determine the localization of inclusions and of larval organs. Orientations of mitotic figures result from flowing movements of the hyaloplasm determined in part by its physical and chemical constitution and in part by outer stimuli. Differences between "regulative" and "mosaic" eggs are due to differences in time and rate of differentiation of the hyaloplasm; in regulative eggs, alternative paths are open and which one is taken depends upon stimuli or "inductions; in mosaic development, only one path is open. Development is one of the most perfect examples of teleology in all nature, but there is no sufficient evidence that teleology in organisms or even purpose in human behavior are not causal phenomena. More and more of the phenomena of development are being traced to mechanistic causes but we are still far from a complete mechanistic explanation of development.