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Periodic nuclear reorganization or endomixis in Paramecium, noted by Hertwig ('88-'89) and since then by many other investigators, and described in detail by Woodruff and Erdmann ('14), is of great interest because of its bearing on questions of parthenogenesis, immortality and nuclear functions in Protozoa primarily, and secondarily in organisms in general. In a series of recent papers Woodruff and Erdmann ('14, '16) and Woodruff ('I7, a and b) have advocated the view that it is a sine qua non in the life of Paramecium, that it is a rejuvenating process analogous to the similar process in conjugation, and that it usually occurs at the low point in the division rate between two "rhythms," the time interval of which is a fixed quantity inherent in the species and which cannot be modified by the external environment. Kasanzeff ('oi), Popoff ('o09), Sun ('I2) and myself (Young, 'I7) have shown that external agents may serve to induce endomixis, and my own experiments show that the frequency of its occurrence may be increased in this way. In my paper (1. c., p. 52) I have stated that " It is more or less cyclical in character and probably has a rejuvenating function. It is not however a necessary accompaniment of temporary depressions and may not be followed by rejuvenation." I have pointed out several cases of the occurrence of low division points without endomixis in Woodruff's and Erdmann's experiments, as well as in my own. I have also indicated one case in their experiments and one in my own in which the process was associated with a rising, rather than a falling division rate; and have shown that in some instances endomixis is not followed by rejuvenation. 38

Mechanisms of gravitactic behaviors of aquatic microorganisms were investigated in terms of their mechanical basis of gravity-dependent orientation. Two mechanical mechanisms have been considered as possible sources of the orientation torque generated on the inert body. One results from the differential density within an organism (the gravity-buoyancy model) and the other from the geometrical asymmetry of an organism (the drag-gravity model). We first introduced a simple theory that distinguishes between these models by measuring sedimentation of immobilized organisms in a medium of higher density than that of the organisms. Ni2+-immobilized cells of Paramecium caudatum oriented downwards while floating upwards in the Percoll-containing hyper-density medium but oriented upwards while sinking in the hypo-density control medium. This means that the orientation of Paramecium is mechanically biased by the torque generated mainly due to the anterior location of the reaction center of hydrodynamic stress relative to those of buoyancy and gravity; thus the torque results from the geometrical fore-aft asymmetry and is described by the drag-gravity model. The same mechanical property was demonstrated in gastrula larvae of the sea urchin by observing the orientation during sedimentation of the KCN-immobilized larvae in media of different density: like the paramecia, the gastrulae oriented upwards in hypo-density medium and downwards in hyper-density medium. Immobilized pluteus larvae, however, oriented upwards regardless of the density of the medium. This indicates that the orientation of the pluteus is biased by the torque generated mainly due to the posterior location of the reaction center of gravity relative to those of buoyancy and hydrodynamic stress; thus the torque results from the fore-aft asymmetry of the density distribution and is described by the gravity-buoyancy model. These observations indicate that, during development, sea urchin larvae change the mechanical mechanism for the gravitactic orientation. Evidence presented in the present paper demonstrates a definite relationship between the morphology and the gravitactic behavior of microorganisms.

This paper deals, with three subjects. Part I sets forth the mode of determination and inheritance of a group of similar traits that occur normally in Paramecium. Part II shows how subjection to a specific reagent brings about permanent hereditary changes in traits of this group. Part III is a study of the differences among the four diff erent hereditary tATpes and is designed to throw light on the nature of thle experimentally, induced changes reported in Part II. I Table I shows how the traits are identified. Four diverse strains (A, B, C, and D) that aroisie within the race 51 ;of Parameiciumin ausrelia (variety 4) wre injected into different rabbits. Later, serum was obtained from each rabbit, diluted with 400 parts water, and tested as to its action by placing some paramecia in a drop of it. Each serum quielcly paralyTzes animals of the strain injected

STUDIES of the sex of Protozoa have been undertaken only in recent years. Contributions'to this problem are as yet few, of which the majority deals with various species of Paramecium. It was Sonneborn (1937) who first discovered the two mating types in P. aurelia. He considered that the macronucleus rather than the micronucleus is differentiated as to sex. Kimball (1937) studied the inheritance of sex in P. aurelia and confirmed the results of Sonneborn, i.e., the sex is determined by the macronucleus. Jennings (1938) investigated P. bursaria and found nine mating types falling into two independent groups. Tartar .and Chen (1940) studied P. bursaria and concluded that it is the cytoplasm which is related to sex. The mating types of P. caudatum have been studied by Giese and Arkoosh (1939) and Gilman (1940). The latter author found from nature five noninterbreeding groups of animals, each with two mating types. The present work was started in May, 1942, using as material P. caudatum of Yunnan, China. Some interesting facts, which have not been recorded by the previous workers, have been observed. These are described in the present paper. MATERIALS AND METHOD

T IHE mating reaction of Paramecium occurs when sexually active animals of opposite mating types are mixed together. Typically, the animals stick, one mating type to another, forming large agglutination masses. These masses break down into smaller clumps, until each consists only of two conjugants of opposite mating type. The conjugants separate after a reciprocal exchange of pronuclei. Numerous investigators have concentrated their attention (predominantly cytological and genetical) on late phases of the mating reaction-conjugation-separation process (for a general review see Sonneborn, 1947); few have examined in detail the steps initiating the process. The principal contributions in this area have been made by Metz and his associates, several of which (Metz, 1946, 1947, 1948; Metz and Foley, 1949; Metz and Butterfield, 1950) are discussed in a later section of this paper.

1. Paramecium caudatum may be easily cut with a scalpel, one, the larger, part, continues to live in about 30 per cent. of cases, and both parts never for more than 24 hours. If the nucleus is injured by the knife neither part lives for more than a few hours. The present paper is a summary of about 150 recorded experiments.2. Cells cut at either extremity are just as much demoralized as those cut in apparently more vital parts.3. The power of regeneration varies in different races of "giant" Paramecium. In one race only about 1 per cent. regenerated; in another race about 10 per cent., in a third race about 30 per cent. regenerated, and in a fourth race all, or 100 per cent. regenerated. This last race is not included in the present paper.4. There is strong evidence of a division zone in Paramecium which lies in the center of the cell. If the cell is cut anterior or posterior to this zone the fragment divides in the original plane into a truncated abnormal form and a normal form. The truncated form may di...

Harper's' criticism of my experiment demonstrating, as it seemed to me, that the anterior end of paramecium is the heavier and consequently that the negative geotropism of this animal is an active process was founded upon the assumption that when one begins to centrifuge the animals their geotropism is reversed and becomes positive. Thus the animals start, according to Harper, to move in the direction of the accelerating force and consequently away from the axis of the centrifuge. Harper's experiment by no means proves his contention and it always seemed to me, as Kanda brings out in the foregoing paper, that the quickness with which a high centrifugal force is developed in the centrifuge would prevent the animals from orienting them selves before they were thrown into the capillary tube where turning is impossible. However, this suggestion might not seem proof to some people; and I have, therefore, performed the following experiment: A culture of paramecium was cooled almost to 00 C. At this temperature they move very sluggishly and show no distinct orientation to gravity. They gather at the bottom of a test tube due, I am sure to the fact that their specific gravity is greater than water and that they make no movement sufficient to keep themselves suspended. The centrifuge was prepared with capillaries such as Kanda and I have used but each one immersed in a larger tube containing ice. A drop of the cold paramecium culture was quickly transferred to the cold capillary tube and centrifuged. The animals were always precipitated with their anterior ends away from the axis. I am quite certain that the animals at this temperature make no reaction to gravity or agitation and that the proof may be considered definite that the head end of the animal is the heavier and that the ordinary geotropic orientation is an active process.