ROUNDWORM ASCARIS LUMBRICOIDES—SCHWARTZ A477 
the two strains show no constant morphological differences. The 
distinction of the two strains on the basis of their host relationship has 
since been supported by the outcome of other experiments, especially 
those involving the feeding to pigs of the embryonated eggs of A. 
lumbricoides, and by epidemiological studies in the American Tropics 
and elsewhere. For instance, Payne, Ackert, and Hartman (17) were 
unable in 1925 to find a correlation between the incidence of Ascaris 
in the pig and man in Trinidad. Despite the close contacts between 
these two hosts and the opportunities for the acquisition of the human 
Ascaris by pigs, the incidence in these domestic animals was only 
about one-seventh of that in man. I made similar observations (21, 
22) 3 years earlier in the Philippines. It should be borne in mind, 
however, that pig Ascaris eggs usually are abundant on the premises 
where these host animals are raised and over which they roam. These 
eggs are, therefore, a potential human health hazard, especially to 
children, even though they may not be able to grow up to maturity 
in the intestines of the human host. 
The available evidence from the numerous experiments that have 
been performed since those that have just been reviewed has estab- 
lished conclusively the fact that A. lumbricoides develops in man 
direct from the embryonated egg to the adult worm. The counter- 
part of this helminth in the pig, to which the name Ascaris suum has 
been applied for convenience, if for no other reason, also has been 
shown to have an identical direct development in the pig. Moreover, 
the evidence at hand supports, on the whole, Koino’s conclusion that 
the human intestine is an unsuitable habitat for the development of 
the pig Ascaris to maturity or to a stage approaching maturity. The 
converse also is true, because the available experimental data indicate 
that the pig’s intestine is an unsuitable habitat for the development 
of the human Ascaris. Moreover, the epidemiological evidence col- 
lected in the United States, in the American Tropics, and elsewhere 
certainly does not support, as previously stated, the idea of a transfer 
of the human or porcine strain of Ascaris to the heterologous host, 
except to the extent that the larvae of either strain can undergo the 
hepatic-pulmonary migrations in either host. 
It is perhaps idle even to speculate on whether man acquired As- 
caris from the pig, or vice versa. Members of the zoological family 
Ascaridae parasitize mammals, such as cattle, horses, pigs, dogs, cats, 
and species of wild carnivores. Ascaris is not known to have a special 
attachment to primates, man’s nearest zoological relatives. Man’s 
helminth parasites are, by and large, closely related to, and in some 
cases identical with, those of the animals he domesticated, and of 
those that have invaded his home as unbidden guests. It is probably 
more logical to assume, therefore, that Ascaris was donated to man by 
the pig than to accept the converse of this proposition, 
