154 CONJUGATION, MATURATION, AND FERTILIZATION 



serves for locomotion (Fig. 66). So, too, in Dallingeria drysdali one 

 of the conjugating gametes has three flagella, while the other has only 

 one. 



Analogous processes occur in Lamblia intestinalis (see Schaudinn, 

 1903), Hexamitus intestinalis, and among many of the phytoflagel- 

 lates, where size difference, however, appears to be facultative. In a 

 number of other cases, however, the adult form is lost during the 

 period of sexual maturity, the organisms becoming ameboid or losing 

 their characteristic motile organs. Thus, in Cercomonas dujardinii 

 and in Tetramitus restrains (Fig. 67) the ordinary firm contour of the 

 body is lost and it becomes highly plastic and changeable in shape, 

 although in the latter the anterior end with the four flagella does not 

 materially change in character until fusion of two cells is well advanced. 

 In Trichomonas intestinalis, on the other hand, the flagella are dis- 

 carded and the body becomes ameboid before fusion (Schaudinn, 

 1903), a condition in which, as Schaudinn observes, it is often difficult 

 to distinguish the flagellate from intestinal amebse. 



(6) The Union of Diminutive Cells. There appears to be no hard and 

 fast line between the phenomenon of union of adults and of smaller 

 cells, for there are cases, especially among the phytoflagellates, where 

 a larger cell may unite with one similar to itself, or with a smaller one, 

 or two smaller ones may unite, and these, in turn, may be similar or 

 dissimilar. Such facultative differences are rarely met with among 

 the animal flagellates, and one consistent rule is usually followed. The 

 union of reduced or diminutive cells is very rare among ciliates, but 

 an interesting exception is the case of Opalinaranarum, where, accord- 

 ing to Neresheimer ('07) the gametes are minute ciliated cells. On 

 the other hand, it is quite common among the rhizopods and seems 

 to be the rule among the foraminifera, but in many cases, as, for 

 example, among the radiolaria, the diminutive cells are at the same 

 time dissimilar, so that they do not properly come under the heading 

 of isogamy. These differences, however, are often so minute that no 

 great value can be placed upon such an artificial distinction. 



Very frequently these diminutive gametes are totally different from 

 the parent cell in mode of locomotion, the rhizopods often forming 

 flagellated gametes which conjugate, the copula developing into the 

 ordinary form. This is the case in Polystomella crispa (see Fig. 52, 

 p. 123), and in Trickospherium sieboldi, Schaudinn ('03); in Pseudo- 

 spora volvocis, Robertson ('05), and in other sarcodina. In other cases 

 an ordinarily motionless form like Gregarina ovata (Schnitzler, '05) 

 and some species of monocystis produce ameboid isogametes. . 



A very interesting case of isogamy has been recently described by 

 Leger ('07) in Ophryocystis mesnili, one of the schizogregarines. 

 Here two cells unite in accouplement, as Leger terms it, a characteristic 

 preliminary union of two gregarines (pseudoconjugation) before 



