ose who were studying
the life-cycle of these parasites were at a loss to know where the
sexual stage took place. In the meantime studies of other parasites more
or less closely related to _Plasmodium_ showed that the sexual stage
occurred outside the vertebrate host. The remarkable work of Dr. Smith
on the life-history of the germ that causes the Texas fever of cattle
had a strong influence in directing the search for this other stage of
the malarial parasite. Another thing that indicated that this sexual
generation must take place outside the body of the vertebrate host was
the fact that the investigators found that the parasites in certain of
the cells did not sporulate as did the others. When these individuals
were drawn from the circulation and placed on a slide for study it was
found that they would swell up and free themselves from the inclosing
corpuscle and some of them would emit long filaments which would dart
away among the corpuscles.

Many men have worked on this problem, but perhaps the most credit for
its solution will always be given to Sir Patrick Manson, the foremost
authority on tropical diseases, and to Ronald Ross, a surgeon in the
English army. There is no more interesting and inspiring reading than
that which deals with the development of the hypothesis by Manson and
the persistent faith of Ross in the correctness of this theory, and his
continuous indefatigable labors in trying to demonstrate it. It was an
important piece of scientific work, and shows what a man can do even
when the obstacles seem insurmountable.

THE PARASITE IN THE MOSQUITO

Briefly stated again, the problem was this: We have here a parasite in
the blood which behaves as do many other forms of life. Some of these
parasites do not go on with their development until they are removed
from the circulation. Now, how are they thus removed from the
circulation under normal conditions? This must first be solved before
the still greater and more important problem of how the parasite gets
from one human host to another can be taken up. In studying this over
Manson reasoned that certain suctorial insects were the agencies through
which blood was most commonly removed from the circulation and he
ventured the guess that this change in the parasite that may be seen
taking place on the slide under the microscope, normally takes place in
the stomach of some insect that sucks man's blood. Ross was greatly
impressed with the theory and beg