THE H^MOFLAGELLATES AND ALLIED FORMS 299 



senting the principal multiplicative phase in the invertebrate host ; 

 it is a form in which the kinetonucleus is placed in front of, or close 

 beside, the trophonucleus, and in which, consequently, the undu- 

 lating membrane is confined to the anterior region of the body, 

 and may be quite rudimentary. As a rule the body of the trypano- 

 monad is shorter, stiffer, more pear-shaped, than in the typical try- 

 panosome-form ; no longer sinuous and flexible, it is held straight 

 and rigid in progression, which is effected almost entirely by the 

 flagellum. In many cases, however, the free flagellum is very short, 

 and used to attach the organism to the lining of the digestive tract. 

 Besides the trypanomonad form, the developmental cycle may also 

 include many other types of form, and often exhibits a degree of 

 polymorphism which is most bewildering, and compared to which 

 the diversity of form seen in the vertebrate host is but slight. 



Taking the development of T. lewisi in the rat-flea as a typical 

 example, the parasites when taken up by the flea pass with the 

 ingested blood into the stomach (mid-gut) of the insect. In this 

 part they multiply actively in a peculiar manner, not as yet de- 

 scribed in the case of any other trypanosome in its invertebrate host 

 (Fig. 130) ; they penetrate into the cells of the epithelium, and in 

 that situation they grow to a very large size, retaining their flagellum 

 and undulating membrane, and exhibiting active metabolic changes 

 in the form of the body, which in early stages of the growth is 

 doubled on itself in the hinder region, thus becoming pear-shaped 

 or like a tadpole in form, but later is more block-like or rounded. 

 During growth the nuclei multiply, and the body when full-grown 

 approaches a spherical form, and becomes divided up within its 

 own periplast into a number of daughter-individuals, which writhe 

 and twist over each other like a bunch of eels within the thin envelope 

 enclosing them. When this stage is reached, the flagellum, which 

 hitherto had been performing active movements and causing the 

 organism to rotate irregularly within the cell, disappears altogether, 

 and the metabolic movements cease ; the body becomes almost 

 perfectly spherical, and consists of the periplast-envelopa within 

 which a number of daughter-trypanosomes are wriggling very 

 actively ; the envelope becomes more and more tense, and finally 

 bursts with explosive suddenness, setting free the flagellates, usually 

 about eight in number, within the host-cell. The products of this 

 method of multiplication are full-sized trypanosomes, complete in 

 their structure, and differing but slightly in their characters from 

 those found in the blood of the rat. They escape from the host-cell 

 into the lumen of the stomach. 



To the intracellular multiplicative phase in the stomach a 

 crithidial phase in the rectum succeeds (Fig. 131). In the fully- 

 established condition the rectal phase consists of small pear-shaped 



