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POPULAR SCIENCE REVIEW. 
or some future period, every fact and opinion of present or past 
times will again be called in question, rediscussed, and dealt with 
according to the knowledge and fashion of the day ; to meet, 
perhaps, the fate that it escaped before of being criticised, anni- 
hilated, and cast into the limbo of past erroneous beliefs, or 
perhaps to come forth re-certificated and raised in significance as 
a golden truth, which has passed unscathed through the trial by 
fire, and shines the brighter for the friction it has undergone. 
This re-sifting of fact and recoil of opinion, is most active 
when a scientific dogma, established on a scarcely secure footing, 
is suddenly brought into collision with unexpected and conflict- 
ing testimony : some discovery, for instance, which is seemingly 
at variance with accepted belief, or which necessitates a new in- 
terpretation of old theory. And such is the actual condition, at 
this present moment, of that branch of physiological science 
which relates to embryogeny, and more especially to the structure 
and properties of u ova.” We purpose, in this article, to make 
our readers acquainted with the facts ascertained by recent 
labours of zoologists, who have given us an account of their re- 
searches. The interpretation of the phenomena to be described 
would lead us beyond our present limits, as it involves a com- 
plete review of the doctrines of embryology. But a brief indi- 
cation of the probable bearing of these discoveries on the 
general doctrine of genesis may, perhaps, prove interesting to 
the general reader. 
The facts, to which we now direct attention, have been inves- 
tigated by several foreign naturalists during the last few years, 
and will be related in order of their discovery. 
1. Professor Wagner, of Casan, found under the bark of a 
dead ash branch a grub, which proved to be the larva of an in- 
sect belonging to the family Cecidomyia. His attention was 
specially arrested by observing that this grub contained within 
its own body a number of similar grubs of different size and 
in different stages of development. He watched their growth, 
from the time of their first appearance as a small organised body 
in the midst of the fatty tissue of the parent, up to the period 
of their complete larval organisation. The presence of larva 
inside the body of another did not at first surprise him, as it is 
a well-known occurrence in the history of certain insects, whose 
eggs and larva have been found inside caterpillars of a different 
species, which have been selected as a temporary home. 
But the larva found and examined by Wagner, contained an 
internal brood of its own species , which was developed from a 
germ mass belonging to itself, and not from eggs deposited by 
an enemy,* though in both cases the unfortunate parent is de- 
* As in the case of the ichneumon fly, which deposits its eggs in the 
caterpillar of the Pieris ; for a good recount of which curious history the 
