574 RONALD ROSS AND R. FIELDING-OULD. 



of gnats (middle intestine) they immeLliately undertake their 

 sexual functions. The male gametocyte (the nucleus of which 

 is larger than that of the female) is destined to give origin to a 

 number of microgametes, or spermatozoa ; the female gameto- 

 cyte develops into one macrogamete, or ovum, together with a 

 residuum consisting chiefly of melanin. A few minutes after 

 ingestion by the gnat both male and female gametocytes 

 break from the enclosing corpuscle, and swell slightly. 

 Attached to the naked parasite one can now often perceive 

 one or two small spherical objects, which may possibly be 

 the homologues of polar bodies. A few minutes later a 

 quivering movement is observed in the male gametocytes, 

 due to the emission of the microgametes. These bodies are 

 long filaments endowed with very active powers of locomo- 

 tion, and consisting of a thread of chromatin surrounded by 

 a thin scroll of bioplasm. Breaking away from the pai'ent 

 cell, and leaving behind the melanin of that cell as a resi- 

 duum, the microgametes travel through the liquor san- 

 guinis contained in the stomach of the gnat in search of a 

 macrogamete. This being found, one microgamete enters 

 the macrogamete and unites with its nucleus, producing a 

 zygote. 



Shortly after the act of fertilisation the zygote may in some 

 species become motile (when it is technically called a vermi- 

 cule), and generally changes its shape. At all events, it travels 

 towards the parietes of the stomach. If the insect be of an 

 inhospitable species the zygote perishes ; but if the insect be 

 hospitable the zygote passes through the parietes and affixes 

 itself on or just under the outer muscular coat of the 

 stomach. Here it becomes motionless and commences to 

 grow rapidly in size. 



At first of about the size of a red corpuscle, and still 

 containing the characteristic black granules of melanin, the 

 zygote, after a week or so, reaches a very large size ; that is, 

 it becomes about 60 /x in diameter, or about eight times its 

 original diameter, and about five hundred times its original 

 bulk. As we have said, we are not satisfied regarding the 



