242 
Annals of the Transvaal Museum. 
In addition to this typical fertilization (the copulation of male and 
female cells), we know of another form, namely, parthenogenesis, where 
the nucleus undergoes a process of self-fertilization. This phenomenon is 
extremely important for the explanation of relapses in protozoan diseases. 
It also plays an important role in the question of immunity. In protozoan 
diseases, in which parthenogenesis of the corresponding parasites occurs, 
we rarely if at all meet with an absolute immunity. It is a relative 
immunity designated immunitas non sterilisans. If we meet with a 
protozoan disease where the female form (macrogametocyte) cannot undergo 
parthenogenesis, we know that in most cases as soon as the man or animal 
recovers, a complete immunity is effected. 
I have given this general review for the better understanding of the 
life-cycle of Theileria parva , and as will be seen later, many facts in that 
cycle, and many conditions of the disease itself, can be explained by the 
biology and physiology of the protozoon. 
In South Africa, the tick which is of chief importance and with which 
I have experimented exclusively in the latter part of my investigations 
is Rhipicephalus appendiculatus. Rh. evertsi , which is also a transmitter of 
East Coast fever, was only used at the commencement of my investigations. 
It is well known that the parasite of East Coast fever does not pass 
through the egg, but the tick can only transmit the diseases in either the 
nymphal or imago stage. For the completion of its cycle the tick requires 
three changes of host. If a larval tick attaches itself on an animal suffering 
from East Coast fever, it leaves the host as soon as it is replete, the length 
of time it remains on the host depending chiefly on the external temperature, 
a fact which is the cause of many difficulties in the study of the East Coast 
fever parasite in the tick itself. After having dropped off the animal, 
the larval tick moults sooner or later, according to external favourable 
or unfavourable climatic conditions. Warmth undoubtedly influences the 
rapid development of ticks. It is only after the tick has moulted into the 
nymphal stage that it seeks a new host, where it again becomes repleted. 
It leaves this second host to moult again for the second time in order to 
finally arrive at the imago stage on the third host. It leaves this third 
host either as a male, which has no other task than to mature and to 
fertilize the female, or as a female to become fertilized and to replete itself 
with a great quantity of blood necessary for the formation of eggs. If 
the tick has been infected as a larva, it can only transmit the disease in 
the nymphal stage ; if it has become infected as a nympha, it can only 
transmit the disease as an imago. 
An infected tick purifies itself completely from all infection once it 
has bitten an animal. If infected as a larva it can only become re-infected 
as a nympha, but never as an adult tick. It only transmits the disease 
in the last stage if it has infected itself in the previous stage as a nympha. 
Its purification can be effected by biting on any mammal ; an ox is not 
exclusively necessary. As far as is known, Theileria parva is only patho- 
genic for cattle. The biological peculiarity in the cleansing of the ticks 
from infection has been made use of in South Africa for the combating 
of East Coast fever. 
