TII CONJUGATION 41 



cumstance that the anterior flagellum of the parent form is unaffected, and 

 becomes without alteration the anterior flagellum of one of the daughter- 

 forms that to the right in the figures. The anterior flagellum of the 

 other product of division that to the left is a new structure formed as 

 an outgrowth from the body : its commencement is shown in C 1 ,^?. i'. 



These two modes of fission longitudinal and transverse both occur 

 in the anchored form of Heteromita, i.e., in individuals attached by 

 the ventral flagellum. The free-swimming form presents a third 

 variety of the process. It comes to rest, loses its regular outline (D 1 ), 

 becoming almost amoeboid in form and finally (D 2 ) globular. Division 

 then takes place : the flagella of the parent become each the anterior 

 flagellum of one of the daughter-cells (compare D 1 , D 2 , and D 3 ), while 

 their ventral flagella are formed by the splitting of a little outgrowth of 

 the dividing body (D 2 , fl. 2'}. 



As in Amoeba fission is invariably preceded by division 

 of the nucleus. 



But in Heteromita fission is not the only mode of repro- 

 duction. Under certain circumstances a free-swimming form 

 approaches an anchored form, and applies itself to it in such 

 a way that the posterior ends of the two are in contact (E 1 ). 

 The two individuals then fuse with one another as completely 

 as two drops of gum on a plate unite when brought into 

 contact. Fusion of the nuclei also takes place, and there is 

 formed an irregular body (E 2 ) with a single nucleus and 

 with two flagella at each end. This swims about freely, and 

 as it does so the last trace of distinction between the two 

 monads of which it is formed is lost, and a triangular form 

 is assumed (E S ), the two pairs of cilia being situated at two 

 of the angles. Still later the protoplasm of this triangular 

 body loses all trace of nucleus, granules, &c., and becomes 

 perfectly clear (E 4 ) : then it comes to rest and loses its 

 flagella, appearing as a clear, homogeneous, three-cornered 

 sac with slightly convex sides (E 5 ). This body, formed by 

 the conjugation of the two monads, is called a zygote, the 

 two conjugating individuals being distinguished as gametes. 



