ABA Prof, R. Leuckart on the Developmental 
from their parents in other respects. They have a more slender 
form (length=0°5-0'6, thickness=0-02-0:023 millim.) and an 
extraordinary mobility. The tail bears a small stiff point. The 
cuticle is distinctly longitudinally striated, and the genital rudi- 
ments in the interior are of small size. 
In this state the young worms remain for a long time, perhaps 
for weeks, without any alteration. They live both in mud and 
water, and also occasionally penetrate into the mollusks (Physe 
and Paludine) which live in company with them. Apparently 
they select the mouth as the starting-point of their wanderings ; 
at least I have repeatedly met with them in the intestines of 
these animals, as well as in the body-cavity. In the latter situ- 
ation the worms have cast their former cuticle and the caudal 
point, and likewise somewhat changed the form of the head. 
This immigration into snails is, however, by no means abso- 
lutely necessary for our worms. The young worms may also be 
converted into the known form of Ascaris nigrovenosa by direct 
transfer to the frog. 
The method employed by me in these experiments was as fol- 
lows. I placed the earth inhabited by the little worms in the 
throat of the frog, and spread it out as much as possible with 
the handle of the scalpel. Direct transfer into the lungs (through 
the glottis and penetrating skim-wounds) did not answer. The 
frogs certainly survived the operation ; but the change occurring 
in consequence of injection in the lungs (strong congestion) 
acted so imjuriously on the parasites, that they could never be 
found a few days afterwards. However, I will not assert that 
the method above recommended led to the desired result in all 
cases. As the frogs soon swallow the introduced earth, most of 
the young worms get into the stomach of the animal experi- 
mented on, where they are certainly to be met with alive for one 
or two days; but finally they all die without undergoing any 
essential alteration. Never more than a few specimens pene- 
trated (through the glottis) into the lungs—at the utmost eight 
to e often only a single one, and not unfrequently none 
at all. 
I presume that the worms usually immigrate into the throat of 
the frog by their own motory powers. As they live in moist 
earth, we might also in this way explain the fact that Ascaris 
nigrovenosa 1s more frequently met with in the land-frog (Rana 
temporaria) than in the water-frog (Jt. esculenta), although the 
latter is as available as the former for the helminthological 
experiment. 
The first changes of the immigrated worms (even of those 
which passed into the stomach) consist in the change of skin 
already mentioned. Within twelve hours after transfer the 
