ROUNDWORM ASCARIS LUMBRICOIDES—SCHWARTZ 469 
over, to ordinary drying because the chitinous layer which envelops 
the shells affords additional protection against desiccation. 
DEVELOPMENT OF ASCARIS EGG 
Let us now turn our attention to the development of the micro- 
scopic Ascaris egg—the starting point in the long and tortuous climb 
that constitutes the life cycle of this helminth. After escaping 
through the uterine opening of the female worm in the host’s intes- 
tine, and becoming incorporated in the intestinal contents, the egg 
is eliminated to the outside with the feces. There it is ready, so to 
speak, to face the perils of a free-living existence. Under favorable 
environmental conditions (a temperature of about 80° to 85° F., free 
access of air, and some moisture), it begins to undergo cleavage (pl. 
1, fig. 1), and in the course of 2 to 3 weeks, a vermiform embryo (pl. 
1, fig. 2), performing more or less constant gliding movements 
within the shell, especially when stimulated by heat, may be readily 
observed even under the low power of a microscope. 
Up to about the middle of the 19th century there was no infor- 
mation on the development of the Ascaris egg, or on the mode of its 
transmission from one host to another. According to the best avail- 
able sources, Gros (8), in 1849, working in Moscow, recorded obser- 
vations on the development of A. dwmbricoides eggs, which he had 
maintained in an incubator at a temperature of 15° to 16° C. from 
the 4th of August to the 2d of December. Three years earlier 
Richter (20) determined that Ascaris eggs remained alive in water 
for a long time and observed, moreover, that those so maintained for 
11 months contained embryos. In 1853 Verloren (26) reared the 
eges of the cat ascarid to the embryonated stage in only 15 days, 
and Heller (9) observed that the embryo casts its sheath while still 
within the shell. It was not until 1857, however, that Leuckart (12), 
at that time undoubtedly the most prominent of all helmintholo- 
gists, made observations on the development of A. luwmbricoides eggs— 
observations which have been sustained ever since. He determined 
that the speed of development varied considerably and was condi- 
tioned principally by the temperature of the environment. During 
the winter months there was little evidence of development, but dur- 
ing the warm summer embryogeny was speeded up to such a point 
that the embryo was already formed in about two weeks. He stated 
also that drying arrested development but the addition of a little 
water reactivated embryonation. 
MECHANISM OF INFECTION 
A logical sequence to the determination of the course of develop- 
ment of the Ascaris egg was to study its mode of infection. The 
