230 THE PROTOZOA 



and stores up reserve-material. Prior to division a sudden and rapid growth 

 takes place at the expense of the reserve-material and by absorption of water ; 

 as a result the protoplasm grows out of the shell-mouth, a daughter-shell is 

 formed, and the animal divides transversely (Fig. 50). 



In Difflugia urceolata. Zuelzer (85) has described a process of chromidiogamy. 

 Two animals come together with the mouths of the shells in contact, and the 

 entire contents of one shell flow over into the other, the empty shell being 

 cast off. The chromidia of the two animals fuse into a single mass ; the 

 nuclei, however, remain separate. Copulation of this kind is a preliminary 

 to encystment, which takes place in Nature at the end of October or the 

 beginning of November. Prior to encystment the pseudopodia are retracted, 

 all foreign bodies, food-remains, excreta, etc., are cast out, and the proto- 

 plasmic body rounds itself off in the shell, and diminishes to about a quarter 

 of its former volume, becoming denser and more refractile. The cyst- 

 membrane is then secreted at the surface of the body. The old nuclei are 

 gradually absorbed, and new nuclei are formed from the chromidial mass. 

 The reconstitution of the nuclear apparatus takes place from January to 

 April ; in the spring the cyst is dissolved, and the rejuvenated Difflugia begins 

 to feed and to enter upon a summer course of vegetative growth and repro- 

 duction. In a recent note (86, p. 191, footnote) Zuelzer states that conjuga- 

 tion between free gametes also occurs in this species. 



In Centropyxis aculeata, according to Schaudinn (131), the ordinary vegeta- 

 tive reproduction is by fission, the new shell that is formed being larger than 

 the old one, until the maximum size is reached. Sexual processes are 

 initiated by degeneration of the primary nucleus, which is single in this 

 species. Then the protoplasm with the chromidia creeps out of the shell, 

 and divides into a number of amcebulse, each containing chromidia which 

 condense into a single nucleus. Some amcebulae form a shell at once ; others 

 before doing so divide into four smaller amcebulae, and then form a shell. 

 The larger are macrogametes, the smaller the microgametes ; they copulate 

 and abandon their shells. The zygote forms a new shell, chromidia appear, 

 and a fresh vegetative cycle is started. 



In a species of the genus Cryptodifflugia (" Allogromia") a remarkable 

 type of life-cycle has been described by Prandtl (265); see also Doflein (7), 

 p. 310, Fig. 283. In this form also the organism, at the time of gamete - 

 formation, quits its shell and penetrates into some other Protozoan organism, 

 such as Amoeba proteus, in the body of which it becomes parasitic and goes 

 through the process of gamete -formation. The nucleus breaks up into 

 chromidia, from which secondary nuclei are formed, producing a multinucleate 

 plasmodium which multiplies by plasmotomy until the host is full of them. 

 Ultimately the plasmodia break up into uninucleate cells, the gametes, which 

 are set free and copulate. The zygote becomes a flagellated Bodo-lika 

 organism, with two flagella, one directed forward, the other backward as a 

 trailing flagellum (p. 270, infra) ; it feeds and multiplies in this form for 

 several generations in the free state, but ultimately it loses its flagella, becomes 

 amoeboid, forms a shell, and develops into an adult Cryptodifflugia. Note- 

 worthy in this development are the alternation of generations between the 

 flagellated and the amoeboid phase, as in Pseudospora (p. 218), and the para- 

 sitism in the gamete -forming phases ; if, however, the Cryptodifflugia does 

 not succeed in finding a suitable host, the gamete -formation may take place 

 in the free state. 



From the life-cycles and sexual processes of Arcella, Difflugia, Centropyxis, 

 etc., it is seen that the primary nuclei of all these forms are vegetative in nature, 

 while the chromidia give rise to the gamete -nuclei, and consist of, or at least 

 contain, the generative chromatin. The marine Trichosphcerium, however, 

 stands apart from the fresh- water genera in regard to its structure, sexual 

 processes, and life-cycle, in all of which it shows more similarity to the 

 Foraminifera. 



