LIFE HISTORY OF ASCARIS LUMBRICOIDES. Di. 
considerably more susceptible to infection than older pigs, but that 
this susceptibility, although diminishing, does not become lost with 
advancing age. 
Whether the lessened susceptibility of pigs to. Ascaris infection as 
they become older is the result of immunity following previous in- 
fections or of increased resistance to infection which comes with ad- 
vancing age is perhaps not altogether certain, but it is hkely that 
age is the determining factor rather than the establishing of an im- 
munity in consequence of an earlier infection. If an immunity is 
established as a result of infection it is not immediately established, 
inasmuch as an experiment on a kid (Experiment No. 23) has:shown 
that two infections may occur with an interval of 17 days between. 
Furthermore, natural cases have been observed in which there were 
adults and smaller worms in the intestine and at the same time 
larval worms in large numbers in the lungs. That age is sometimes 
an important factor in the occurrence of parasitic infection is indi- 
cated by experiments by the senior writer with gapeworms (Syn- 
gamus trachealis) which have shown that chickens become less sus- 
ceptible to infection as they grow older, adult chickens rarely har- 
boring the parasites. Various observers have noted in the case of 
other parasites that young animals are more susceptible to infection 
than older animals, and it is evident that age often has a great deal 
to do with susceptibility to invasions by parasites. On the other 
hand, in the case of some parasites and some species of host animals, 
no immunity develops with increasing age. For example, although 
there is a great reduction in the susceptibility of chickens to infec- 
tion with the gapeworm as they become older, there is no marked dif- 
ference in the susceptibility of turkeys of different ages to infection 
with the same parasite. 
The question of the production of immunity to Ascaris infection as 
the result of earlier infections requires further investigation. Our 
present knowledge, however, indicates that as pigs grow older they 
become less susceptible to Ascaris infection, not because they have 
been immunized by earlier infections, but because with advancing 
age they become more resistant to infection. 
ASCARIS PNEUMONIA. 
As first shown by Stewart (1916a), the invasion of the lungs by 
Ascaris larve may cause a serious, sometimes fatal, pneumonia. Rats 
which had been fed Ascaris eggs became ill with a pneumonia four to 
six days afterwards and on post-mortem examination numerous 
Ascaris larve were found in the lungs. In later papers Stewart also 
notes the occurrence of pneumonia in mice and pigs following the 
administration of Ascaris eggs. 
