114 
The Rot in Sheep. 
conjecture to some extent takes the place of observation Judging, 
however, from analogy with regard to the development of other 
Trematoda, there appears no reason to doubt that the ciliated 
embryo of the Distoma hepaticnm does not undergo any material 
change until becoming parasitic to water-snails, slugs, &c., and 
that when thus located it becomes converted into a peculiar 
organism called a Cercaria-snc [see Jiff. 10, paffe 115). From the 
nucleus of the distoma-embryo development goes on, and a brood 
of young Cercaria; are ultimately formed within the sac, by 
a species of successive budding, each one in turn thus be- 
coming a parent. From the first, second, or third of these 
offspring a return to the form of the original parent distoma 
takes place. 
This system of propagation has been described most accurately 
by Steenstrup, who has named it " Alternation of Generation,'^ as 
differing materially from ordinary metamorphoses. We give his 
own definition of the process : " Alternation of Generation is," 
he says, " the remarkable phenomenon of an animal producing 
an offspring which at no time resembles its parent, but which, 
on the other hand, itself brings forth a progeny which returns in 
its form and nature to the parent animal ; so that the maternal 
animal does not meet with its resemblance in its own brood, but 
in its descendants of the second, third, or fourth degree of 
generation." * 
Many examples of this system of propagation take place in 
nature, and among creatures far higher in the scale of organisation 
than those of which we are now speaking ; but it is unnecessary, 
in a treatise of this kind, that these should be furnished. We 
may, however, direct the reader seeking such information to 
Steenstrup's work, and also to Professor Owen's on Partheno- 
genesis, Kiichenmeister's on Parasites, t Von Siebold's on 
Cystic Worms. % 
The CercaricB, so called from their caudate form (see Jiff. 12 
J)affe 116), were for a long time considered as Infusoria when found 
to be floating freely in water, their origin and mode of propa- 
gation being unknown until the discovery of Steenstrup. The 
cercaria-sacs were designated by him "nurses," and the young 
cercarice developed within them '■^parent-nurses" — terms which 
have helped rather to mystify the matter than to render it plain. 
Most cercaria-5a.cs are of simple organisation ; but, notwithstand- 
ing this, they are found of various forms, according to the kind 
of cercarice to be developed within them. 
* ' Alternation of Generations,' by J. Japetus Sm. Steenstrup, translated from 
the German by George Busk. London, 1845. 
t Translated by Dr. Lankester. % Translated by Professor Huxley. 
i 
