Aging eggs show progressive measurable morphologic and physiologic changes. Besides those described in Study II., there are a number particularly evident in later stages of aging eggs, such as agglutination and fusion of eggs, irregular cleavage, separation of the blastomeres and cytolysis. A. 1. The agglutination phenomenon occurred only in physiologically very deteriorated or "aged" eggs, as evidenced in part by the low per cent. of the jelly layer, inability to develop a fertilization membrane, a more viscous condition of the cytoplasm and cortical layer, and by numerous other tests. These are exactly the conditions that have been sought in the experimental agglutination of eggs. 2. The onset of agglutination occurred at different ages for the eggs of different females, such variations being due to differences in physiologic condition of the eggs at the time of liberation from the parent. The more deteriorated the eggs at liberation the earlier the agglutination, and vice versa. Agglutination occurred in every experiment in which the eggs had aged sufficiently. 3. Some eggs remained agglutinated throughout their subsequent developmental history, others were secondarily separated at varying swimming stages in development, others died precociously, due to, asphyxiation. 4. Fertilized as well as unfertilized eggs were agglutinated. 5. Clusters of 2 to 40 or more eggs were thus agglutinated. 6. Agglutination is not determined by the condition of the sperm except in so far as concentrated fresh sperm may, by revolving the eggs, separate agglutinated eggs or prevent their so doing. B. 1. When the eggs were in sufficiently poor physiological condition, as determined by suitable tests, they tended not merely to agglutinate but many subsequently fused more or less completely. 2. Such fusion may occur in the egg stage or during subsequent development, with corresponding complete or incomplete fusion. The degree of fusion determined the various types of fused embryos and larvæ described in previous publications. 3. Complete spontaneous fusion of not more than three eggs occurred not infrequently and gave rise to giant eggs. 4. Fusion occurred in all experiments in which the eggs were allowed to age sufficiently. 5. The variation in the time of fusion was determined by the physiologic condition of the eggs at liberation. The more deteriorated the sooner the fusion. 6. Fused eggs and embryos, and giant eggs occurred in all three species of sea-urchin eggs, namely, Arbacia, Hipponoë and Toxopneustes. 7. It is probable that the "spontaneous" fusions of larvæ and embryos, described by many workers, find their explanation in similar aging or deterioration of the eggs. C. 1. With increasing age and physiologic deterioration of the eggs, cleavage was increasingly irregular. This irregularity was manifested in a change in size and shape of the blastomeres, in retardation in the rate of cleavage, in increasing inhibition at progressively earlier stages of development, increasing numbers and types of atypic embryos and larvæ, and in extreme stages, in total lack of cleavage. 2. In every batch of eggs irregular cleavage occurred as soon as the eggs had aged sufficiently. The onset and the degree of irregularity varied with the physiologic condition of the eggs when liberated. Once begun there was a progressive increase in the numbers and in the degree of abnormality. 3. Irregular cleavage which is a consequence of aging is due in part to excessive intake of sea water, and in part to polyspermy. The quantitative relations of these two factors was not determined. Both are due to a change in the cortical layer of the egg. 4. Deteriorated or irregularly cleaving eggs, were shorter lived than physiologically, fresh eggs. The greater the irregularity (or physiologic age) the short lived the eggs, and vice versa, the more virile the egg the greater the longevity and the less irregular development. D. 1. With increasing physiologic deterioration, there occurred another type of irregularity, namely, more or less complete separation of the blastomeres. 2. Such separation occurred in every experiment that was carried over a sufficiently long period, and with further aging separation of blastomeres was increasingly complete. 3. The separation of the blastomeres occurred with successive cleavages. 4. In Toxopneustes and Hipponoë, separation usually occurred eight to ten hours after physiologically good eggs were removed from the female. In Arbacia about 24 hours were required. The variation in onset in different females was due to variation in the physiologic condition of the eggs at liberation. E. 1. With still further physiologic deterioration (or "ageing") cytolysis set in. There were two types of cytolysis, namely cytolysis by liquefication or enlargement, and by fragmentation or reduction. 2. The onset of cytolysis differs in the eggs of different females depending upon the physiologic condition of the eggs at the time of liberation. 3. Cytolysis occurred three or more times as rapidly in Toxopneustes and Hipponoë than in Arbacia. This difference is in part due to accelerated metabolic rate at the higher temperature of the southern species, partly due to increased HO ion concentration of sea water at Tortugas and partly to protoplasmic differences of the different species of eggs. F. Agglutination, fusion, abnormal cleavage, separation of blastomeres and cytolysis of the eggs are phenomena correlated with intense physiologic deterioration of the eggs, and were observed in all cultures in which the eggs were sufficiently deteriorated. Seven independent groups of tests were used to determine the degree of deterioration. These tests corroborate one another. Any one test, with suitable precautions, measures the vitality of the eggs. From any one, the other manifestations of aging may be predicted. A group of tests offers the most convincing means of measuring exactly the degree of senescence of any sample of of eggs. These seven groups of tests or symptoms of senescence also indicate the fundamental nature of the chemical and physical changes involved in the aging process, namely a change in the cortical layer of the egg, and a change in oxidation. The change in the cortical layer, i. e., the change in permeability, affects the membrane, the cleavage, and all the other consequences of aging above enumerated. It is very probable that the changed cortical layer with its train of consequences, as well as the change in respiration, are fundamentally reducible to the one phenomenon, namely, changed metabolism. These results afford a common explanation of apparently diverse phenomena, such as change of sex with age (Riddle, Hertwig, etc.), senescence (Minot), reduced productivity (Pearl, King, etc.), physiologic differences in cross fertilizations (Tennant, etc.), etc. (For details see discussion.)