THE MASTIGOPHORA 165 



division of the nucleus. The number of chromosomes seen in these 

 mitotic divisions is about 40, and there are no centrosomata at 

 the poles of the spindle. The sexual reproduction is preceded 

 by the formation of mega- and microgametocytes. In the early 

 stages the gametocytes cannot be distinguished from the ordinary 

 vegetative individuals except as regards the microscopic character 

 of the nuclei. A number of minute granules of chromatin 

 (chromidia or sporetia of Goldschmidt) are extruded from the 

 nucleus, increase in number and size, and give rise to the nuclei 

 of the numerous gametes. The cytoplasm of the gametes is 

 formed by a differentiation of clear protoplasm around each 

 nucleus. In the case of the formation of the megagametes at least 

 one mitotic division of the nucleus occurs, which has been inter- 

 preted to be a polar division. A similar polar division of the 

 nucleus probably takes place also in the formation of the micro- 

 gametes. The elements of these nuclei are so small that it has 

 not been proved that a definite reduction in the number of the 

 chromosomes occurs. In both kinds of gametocytes an encystment 

 accompanied by withdrawal of the pseudopodia and flagella occurs, 

 but the microgametocyte encysts sooner than the megagametocyte. 

 The gametes escape from the gametocytes and conjugate to form 

 a zygote. The megagametes are about 3-6 //, in diameter and 

 are provided with a single flagellum 15-18 /z in length. The micro- 

 gametes are 2-8 /* in diameter and are also provided with a 

 Hagellum. The zygote is a minute active monad, which divides 

 several times by simple fission and then grows and assumes the 

 general characters of the genus. 



The principal genera are : Amoeboid and monomastigote : Mastiga- 

 moeba, Schulze ; Mastigella, Frenzel ; Mastigina, Frenzel. Amoeboid and 

 dimastigote : Cercobodo, Kent = Dimastigamoeba, Blochmann, and some of 

 the species attributed to the genus Cercomonas (Fig. 5 (32, 33)). Heliozooid 

 and monomastigote : Actinomonas, Kent. Heliozooid and dimastigote : 

 Dimorpha, Gruber. 



TRIBE 2. PROTOMASTIGINA (sensu stricto). 



Solid food is ingested at a fixed point near the base of the flagellum. 



SUB-TRIBE 1. MoNOMASTiGODA. 1 A. Flagellum directed forwards. 

 a. Oicomonas, Kent (Fig. 5 (29, 30, 31)). Ingestion of food at base of 

 flagellum by means of a protuberant vacuole (vacuolar ingestion) which 

 subsequently migrates to the posterior end. /3. Leptomonas, Kent. Rod- 

 shaped or fusiform, parasitic in intestine of insects. B. Flagellum 

 directed backwards, a. Ancyromonas, Kent. The single flagellum arises 

 at anterior end, but is bent backwards and serves as an anchor or 



1 This sub-tribe comprises the Cercomonadina of Saville Kent or the Oicomona- 

 daceae of Senn. From the work of Klebs and others it seems necessary to reject 

 the genus Cercomonas, since the confusion surrounding it cannot be lightened. 



