456 Prof. R. Leuckart on the Developmental 
extremity of which we may already detect distinct ova, of 0:01 
millim. in diameter (germinal vesicle = 0:007 millim.). In 
worms of 3°5 millims. these ova have attained nearly their full 
size (length=0-08, thickness = 0-085, germinal vesicle= 0-028, 
germinal spot =0:0085 millim.), and a granular yelk-mass has 
accumulated in them. They are contained in the genital tube, 
which measures about 8 millims. and is strongly folded together, 
and have not yet passed into the uterus, which is still short, 
and are destitute of shells. Perfectly developed eggs with shells 
are first seen in individuals of 5 millims. When the length has 
reached 6 millims., the uterus, which has in the meanwhile con- 
siderably increased in size, not only contains numerous mature 
ova, but also ova in all stages of embryonic development, and 
even perfectly mature embryos. 
It is, however, exceedingly remarkable that the immigrated worms 
always develope only into female individuals. Even the preg- 
nant animals bred by me were always unaccompanied by males. 
As I have nowhere else met with male specimens of A. ngro- 
venosa, although I have examined many hundred females, and 
have never found a trace of semen in the sexual organs, I have 
no hesitation in regarding the lung-worm of the frog as a 
parthenogenetic creature. That this is the first case of the kind 
among the Nematoda, or indeed among the Entozoa in general, 
can no more invalidate my supposition than the fact that older 
writers occasionally mention the male of Ascaris migrovenosa. 
What they (Zeder, &c.) state about these so-called males by no 
means proves their existence, and leaves room for the supposition 
that younger specimens have been regarded as males. 
How widely the developmental processes here described for 
A. nigrovenosa may be diffused among the Nematoda can only 
be ascertained by further investigations. At any rate, it would 
be too precipitate, starting from the preceding facts, to regard 
all the numerous Ahabditis-forms of mud and dung as mere 
developmental stages of parasitic roundworms. I am acquainted 
with nearly thirty different species of this group, but have not 
met with a single one which I could with any probability bring 
into the developmental cycle of a parasite. 
However, I am not the first to assume alternating genera- 
tions of parasitic and free forms for certain Nematoda. About 
ten years ago, an excellent naturalist, Carter, of Bombay, asserted 
that the embryos of the notorious Filaria medinensis did not be- 
come developed into parasites, but into free-living Rhabditiform 
worms (Urolabes, Carter), which grow to sexual maturity in 
water, and only resume a parasitic existence in their progeny. 
That doubt should have been repeatedly thrown upon this state- 
ment is perfectly explicable, considering the want of all objective 
