320 PARASITES OF THE DOMESTIC ANIMALS 



If the blood is now di-awn by an anopheline mosquito further changes 

 take place-. 



The Sporogonic or Sexual Cycle. — In the intestine of the mosquito 

 the female gametocyte undergoes certain nuclear changes preparatory 

 to fertilization; the cell becomes rounded or oval in form, and is now 

 known as the macrogamete. From the male gametocyte there are ex- 

 truded from three to six fiagelliform filaments corresponding in number 

 to the peripheralh' disposed daughter nuclei. These filaments detach 

 from the mother cell to become the actively motile microgametes, which 

 are analogous to the spermatozoa of higher animals. Thus the flag- 

 ellated parent body maj' be referred to as a microgametoblast; produc- 

 ing the male sexual elements or microgametes. 



Fertilization of the macrogamete is brought about by its penetration 

 by one of the microgametes. The fertihzed macrogamete now becomes 

 the ookinete or zj-gote, in which stage it passes by a vermiform move- 

 ment into and through the epithelium of the mosquitoe's mid-intestine 

 and comes to rest just beneath the outer lining membrane. Here it 

 rapidly grows, the nucleus divides, and by the third to the fifth day it has 

 formed a cyst in which there are many nuclei, each to become the nucleus 

 of a minute body, — the sporoblast. The sporoblasts, by division, form a 

 number of germs, — the sporozoites, spindle-shaped, nucleated bodies 

 which are mature after a period of ten to fourteen days in the body of 

 the mosquito. On reaching maturity, the sporozoites are liberated into 

 the body cavity of the insect where they are carried about b}^ the body 

 fluids, collecting eventual^ in the salivary glands. From here they 

 pass to the piercing proboscis from which, with the next bite of the mos- 

 quito, many may pass into the blood of another human victim to begin 

 the asexual cvcle. 



Plate IV.^ — Life Cycle of the Malaria Parasite. 1. Free sporozoite, either in 

 salivary glands of the mosquito or in blood of man. 2. Penetration of the sporozoite into a 

 red blood corpuscle. 3 to 6. Growth of trophozoite. 7, 8. Division of trophozoite which 

 brings about destruction of the blood corpuscle and the release of the merozoites in the 

 blood stream. The free merozoites then enter new blood corpuscles, and this cycle may 

 be repeated many times. Finally, however, the sexual cj'cle is initiated as follows: 9a to 

 12a. Growth and differentiation of female cell. 9b to 12b. Growth and differentiation of 

 male cell. 1.3a, 13b. The male and female cells are swallowed by a mosquito. 14a. Matu- 

 ration of female cell. 14b. Formation of microgametes. 15b. Free microgamete. 16. 

 Fertilization. 17. Ookinete. 18, 19, 20. The ookinete attacks and penetrates a cell of 

 the intestine of the mosquito, and passes completely through the epithelium, coming to 

 rest in the peri-intestinal tissue. (There is not, in life, the reduction in size indicated by 

 the figure.) 21 to 25. Stages in the development of the cyst and formation of the sporozo- 

 ites. 26. Migration of the sporozoites. 27. Sporozoites in the salivary glands of the 

 mosquito. 13c to 17c. These figures portray the cycle which is supposed to account for 

 cases where malaria is latent for a longer or shorter period. Ordinarily, unless removed Ijy 

 a mosquito, the differentiated male and female cells (12a and 12b) die, but under certain 

 conditions the latter may continue to live in the blood, to give rise to a renewal of the 

 disease. (After Crawley, from Mense's "Handbuch," after Grassi and Schaudinn, Cir. 

 No. 194, Bu. An. Ind., U. S. Dept. Agr.). 



