1917] Glaser—Growth of Insect Blood Cells in Vitro 3 
The inoculated slides showed no indications of the formation of 
polyhedral bodies within the nuclei of the blood cells. This ex- 
periment was repeated twice more and with the same result; no 
difference between the experiments and checks was observed. 
Twelve healthy M. americanum larvee were fed with polyhedral 
virus passed through Berkefeld Grade “N” filters. As checks the 
same number of larvee were infected with the virus sterilized by 
autoclaving. At the end of ten days tissue culture preparations 
were made with blood taken from the experimental animals and 
from the checks. The slides were studied at once and it was found 
that two thirds of them, representing blood taken from animals 
fed with the unsterilized virus, showed infection. The early stages 
of polyhedra were discernible within the nuclei of many of the 
blood cells. Other cells still seemed to be in a normal condition. 
The slides representing blood taken from the checks appeared 
perfectly normal. The next day all of the slides were again 
examined, but no change was noticed with the exception that some 
of the cells had divided. In one day more nearly all of the ex- 
perimental blood cell nuclei were beset with large and small poly- 
hedra. In six to seven days the blood cells from the experimental 
animals began to disintegrate with the liberation of small and large, 
well formed, typical polyhedra. The cells on the check slides . 
disintegrated normally five days later. 
A large number of the blood cells of M. americanum are of the 
mulberry corpuscle type (PI. I, fig. 2). These are not so well 
adapted to cultivation as the ordinary blood cells (PI. I, figs. 3 and 
4). For this reason the experiments were repeated with the blood 
of Porthetria dispar in which the mulberry cells are in the minor- 
ity. In these experiments it was likewise impossible to infect 
growing blood cells with the polyhedral virus, but if animals were 
first infected the formation of the polyhedra could be traced very 
nicely by taking the blood from the infected animals in about ten 
or twelve days and studying by means of the tissue culture method. 
What do these experiments signify? Several possibilities at 
once suggest themselves, but I will merely outline the two most 
probable. First of all let us suppose that I have actually cultivated 
the polyhedral virus on the tissue culture slides. Then why is it 
impossible to infect such tissue directly with the virus? Why is it 
necessary to give the virus “a start” within the insect itself? Per- 
