[17] ARTIFICIAL PROPAGATION OF FISH. 507 
Water rats are also extremely dangerous to the embryo. They may 
be destroyed (though with considerable difficulty) by means of wire 
traps. In order to prevent as much as possible the approach of these 
unwelcome guests one should be careful not to destroy the damaged 
eggs near to the hatching apparatus, as thus the odor of putrefying 
animal matter which attracts the rats will be avoided. 
We must once more direct attention to what has been said above re- 
garding the temperature of the water. We believe that, if the water 
in the incubating apparatus is kept at the temperature which we have 
given above as being most suitable for fecundation, the first condition 
for a natural and rapid development is fulfilled. The person in charge 
of the apparatus should always watch the thermometer; by opening or 
closing the apparatus, or by adding cold or hot water, an even degree 
of temperature may be maintained. 
§ 3.— Transformation and development of. the egg. 
There are various changes in the appearance of eggs which have been 
fecundated; one might almost say that their contents become turbid and 
that they become less transparent than when they left the opening near 
the anal fin; they again, and almost imperceptibly, assume their first 
transparent appearance, whilst in the inside there may be observed a 
small circular spot which was not seen there at first* (Fig. 22,1). This 
change has erroneously been considered a certain sign of fecundation; 
it takes place both in fecundated and non-fecundated eggs, but it de- 
velops more slowly and irregularly in the latter (Fig. 22, 2). 
During the first moments and even during the first days it is (with 
certain kinds of fish) impossible to distinguish with the naked eye fee- 
undated from non-fecundated eggs, but when examined under the micro- 
scope there can no longer be any doubt. 
After a certain time an arched line makes its appearance in the fec- 
undated eggs (Fig. 22; 3, 4,5). This time differs not only according to 
the different species of fish to which 
the eggs belong, but also accord- § 
ing to the temperature of the water 
in which they are placed. Farther 
down we shall point out the causes 
of this (Fig. 22). 
This line increases in size with the gradual development of the eggs; 
whilst one of its ends is prolonged in the shape of a tail, the other as- 
sumes the form of a spoon. This latter part is the future head of the 
young fish; the eyes, which now appear like two brown pointst (Fig. 22, 
6), prove this sufficiently. 
The motions, particularly of the tail, of the young fish become more 
Fic. 22. 
* This fact has already been observed by ARISTOTLE. Hist. anim. ., lib. 6, cap. io 
t Also noticed by Aristotle. 
