MORPHOLOGY, LIFE CYCLE AND PHYSIOLOGY OF P. V1VAX 
33 
to more reproductive forms, with the re¬ 
sult that in certain individuals the infec¬ 
tion may persist for three years * 1 11 or even 
longer, when others exhibit no capacity for 
reproduction at all, and develop into game- 
tocytes ? 
Gametogeny 
This question cannot be satisfactorily 
answered at present, but Boyd (1935b) has 
made an interesting study which may 
throw some light on the problem. He be¬ 
lieves that parasites occurring in the blood 
may be divided into five series, differing 
morphologically in minor ways. From one 
series, into which most forms fall, come 
merozoites of three types. One type, which 
is produced in the largest numbers, is des¬ 
tined to go on and repeat the reproductive 
cycle. The two other types also go through 
the process of schizogony, but the mero¬ 
zoites they produce become gametocytes— 
i It is believed that vvvax infections do not per¬ 
sist more than three years in default of fresh infec¬ 
tion, even when untreated. Exceptionally they may 
possibly persist longer. 
the male forms arising from one type and 
the female from the other. The develop¬ 
mental stages of each of the latter make up 
the other two series. Fig. 19 is taken 
from his paper, but with some changes to 
indicate the relationship of the series to 
one another. It may be noted that although 
parasites of the different series differ some¬ 
what in morphology, all of them require 48 
hours for maturation. 2 The factors which 
cause certain merozoites to develop in one 
direction rather than another still remain, 
however, quite obscure. 
2 In this regard Boyd’s conclusions are not in 
accord with the generally accepted belief that 
gametocytes require 96 hours for development. 
Boyd also expresses the opinion that the sexual 
forms probably do not live more than 24 hours. It 
is difficult to devise experiments to determine ex¬ 
actly the life of the sexual forms, but it has been 
thought that they live considerably longer than this. 
It is also usually stated that the gametocytes go 
through a large part of their development in the 
capillaries of the spleen and bone marrow ( cf . 
Wenyon 1926; Manson-Bahr 1931). The relation¬ 
ship of the gametocytes to forms belonging to the 
pigment-free cycle is also a problem which may need 
future investigation. 
Explanation op Plate I 
1. A very young parasite. 
2. A somewhat older form. The characteristic pseudopodial processes are very evident. 
3. A trophozoite of moderate size. The enlargement of the host-cell, which is invariable with this species, 
is already evident. 
4. A trophozoite showing the large vacuole which very frequently characterizes growing parasites of 
this species. 
5. Here the vacuole gives the parasite the appearance of a large ring. Smaller rings are also very 
often seen. In this parasite, and also in those shown in Pigs. 4 and 6, the three granules 
which, according to Boyd, are characteristic of forms destined for ordinary schizogony, may 
be seen. (See Series A in Pig. 19, Plate II.) 
6. A large trophozoite exhibiting conspicuous pseudopodial processes. 
7. A young schizont in which the nucleus has divided once. 
8. An older schizont. The two chromatin masses on the left are in division. In the original prepara¬ 
tion the division figures were very clear, and resembled spindles, although nothing having the 
appearance of chromosomes could be seen. 
9. A mature schizont containing apparently 17 merozoites. Sixteen is generally regarded as the typical 
number. 
10. A trophozoite probably destined to become a gametocyte. The Sehuffner’s dots are very conspicuous. 
11. A macrogametoeyte. Note the rather small and compact nucleus. 
12. A mierogametocyte next to a schizont. The large and relatively diffuse nucleus is very clear. 
13. A microgamete. 
14. Sporozoites. 
15. Sporozoites within the salivary glands of a mosquito. Careful observation will show several with 
a divided nucleus (indicated by arrow). 
Magnification in Pigs. 1-10, 12, 15 is 2000 diameters; in Pigs. 11, 13, 14 it is 2700 diameters. 
Pigs. 11, 13 and 14 are from mierophotographs by Dr. Mark P. Boyd, to whom the author desires 
to express his appreciation for their use. Pig. 12 is copied from one of Dr. Boyd’s photographs. The 
others are from the author’s own preparations and were made by Miss Stella Zimmer of the Department 
of Photography, Syracuse University Medical School. 
