450 Prof. R. Leuckart on the Developmental 
it is only the length that increases. The transverse section of 
the body remains almost unchanged, and thus the worm gradu- 
ally alters its original form for a more slender one, at the 
same time increasing in mobility in the same proportion. The 
pharynx and tail retain their previous form, and undergo but 
little alteration in size, whilst in the periphery of the buccal 
orifice three small papille gradually sprout forth, and a few 
(usually four) oil-drops accumulate in the body-cavity on each 
side behind the anterior pharyngeal dilatation, and, remaining 
pretty regularly grouped, produce almost the impression of eyes 
destitute of pigment. 
In this state I have sometimes met with the young worms in 
the nasal cavity and rectum of the frog; so that one might be 
led to suppose that they may be converted, after immigration, 
directly into the definitive form. The experiments made by me 
in this direction, however, produced no results. Once, certainly, 
the young worms were found still unchanged in the rectum on 
the sixth day after their transfer; but otherwise they seem 
generally to die pretty quickly. 
The example of Ascaris acuminata shows us, even more 
strikingly than that of Dochmius, that the free young states of 
the Nematoda not unfrequently attain a high degree of indepen- 
dency. But A. acuminata by no means reaches the final limit 
of this development. There are Nematoda the embryos of which 
even attain sexual maturity in their Rhabditis-form, and only 
become parasitic again in their progeny— Nematoda, consequently, 
the history of which presents us with no simple alternation of 
the conditions of life, but with an alternate sequence of free and 
parasitic generations. And, what is most wonderful, both these 
generations are sexually developed—both are produced from ova. 
Here, therefore, we have nothing to do with an ordinary alter- 
nation of generations, such as occurs, for example, in the Dz- 
stomee, but with a process hitherto almost unheard of in the 
animal kingdom, and which calls for our consideration the more, 
because we are accustomed to regard the sexual development of 
an animal not merely as the sign of its perfect maturity, but 
also as the criterion of specific individuality. 
The roundworm which undergoes this peculiar development 
is one which has been repeatedly investigated—the well-known 
Ascaris nigrovenosa of the lungs of our brown frog (Rana tem- 
poraria). 
The embryos of this Nematode worm*, as everyone knows, 
* Tn the investigation of the life-history of Ascaris nigrovenosa I was 
gratified by the participation of a talented young zoologist, M. Elias 
Mecznikow, of Charkow, who has also taken a lively interest in my other 
observations and experiments upon Nematoda. With regard to the pre- 
