42 ZOOLOGY FOR MEDICAL STUDENTS chap. 



able to swim rapidly. The first type is the macrogamete, female gamete 

 or egg : the second is the microgametej male gamete, or spermatozoon. 

 The yolk or food-material is concentrated in the non-motile female 

 gamete and this leads to these gametes being large in size and there- 

 fore few in number ; while on the other hand the male gametes are 

 small in size and developed in great numbers. 



To prevent inbreeding micro- and macrogametes are not developed 

 in the same colony at one time. They are either developed in separate 

 male and female colonies or the sexual colony is at first male — producing 

 microgametes — and then becomes female later on. 



Having passed in review the main features of three illustrative genera 

 of flagellates as seen from the point of view of pure science we will now 

 proceed to study certain genera which are of special practical importance 

 from their having adopted a parasitic mode of life and being thereby 

 in some cases seriously harmful to the welfare of the host in whose body 

 they have taken up their abode. 



Trypanosoma 



Animals belonging to the genus Trypanosoma seem to have been 

 observed as far back as the year 1843 when specimens were found in the 

 blood of the frog by Gruby. It is only within the last two decades 

 however that trypanosomes have excited very special interest owing to 

 the fact that a number of important diseases have been traced to their 

 agency. 



A typical trypanosome (Fig. 18) is elongated and somewhat spindle- 

 shaped, one end being rather more pointed than the other. The pointed 

 end is prolonged into a single flagellum (/) which is continued back along 

 the side of the body for some distance, it may be throughout almost its 

 whole length. The flagellum from its basal end forwards to the tip of 

 the body lies at some little distance from the surface of the latter, being 

 connected with it by a thin protoplasmic membrane (Fig. 18, m). This 

 membrane looks like a kind of fin running along the body, the flagellum 

 forming its thickened free edge : the membrane has a frilled appearance, 

 the flagellum along its edge following a somewhat sinuous course. The 

 cytoplasm of the trypanosome is finely granular : and superficially it is 

 slightly stiffened to form a bounding layer. The nuclear apparatus is 

 very characteristic. Somewhere about the middle of the creature is a con- 

 spicuous nucleus (Fig. 18, t), rounded or oval in form, with a dense central 

 mass of chromatin (karyosome) and smaller masses round the periphery. 



