212 Bibliography. 
' the author, commencing with the earliest period at which the ovum 
comes within the reach of the microscope, and following it through its 
. various stages of development, terminates with the escape of the em- 
bryo from its membranous envelopes. 
The arrival of the spawning season of the Coregonus pala, the spe- 
cies experimented upon by M. Vogt, is announced by these fishes asso- 
Clating in pairs, and jumping frequently above the surface of the water, 
the female protruding ova at the same time that the male ejects the 
seminal fluid; by these simultaneous movements, impregnation is effect- 
ed, but large numbers of ova escape the fecundating influence. In or- 
der to procure a sufficient number of ova for microscopic observation, 
he has found it most convenient to have recourse to artificial impregna- 
tion, which is more successful, as regards the numbers impregnated, 
than when effected in the natural manner. The artificial mode con- 
sists simply in squeezing ova from the female, and the seminal fluid 
from the male, and the two are brought in contact by being placed in a 
vessel containing water. The development takes place perfectly well 
in the house, provided water is used taken from the locality in which 
the fish usually deposits its ova. This last is by no means an unimpor- 
tant step, since his experience goes to show that ova which are ordina- 
rily deposited in lakes, are destroyed or blighted, if water from rivers 
is made use of, and vice versa. The water should also be frequently 
agitated and changed, and all sudden variations of temperature avoided ; 
the most favorable degree of warmth, being from 4° to 8° R. or from 
40° to 50° F. Congelation of the water in which they are contained, 
retards their progress, but does not destroy life. 
He finds the ova liable to diseases, and like those of some species of 
Limax, are often attacked by vegetable growths, consisting of slender 
jointed filaments, often expanded at their free extremities, giving the 
egg a milky appearance, and finally destroying its vitality. He has 
seen the same growths attacking young fishes, and causing death in the 
course of eight or ten days. If the same specimen is to be brought 
frequently under the microscope, care should be taken to keep it im- 
mersed in water, and not to keep under the instrument beyond a few 
minutes at one time. If the outer or shell membrane is to be removed, 
it is most easily accomplished under water; if the vitelline membrane 
is to be opened, this should be done in the air, for the vitellus is ren- 
dered completely opaque, by being brought in contact with water. 
We mention these different steps, for the benefit of those who wish to 
engage in similar investigations. 
If the ovary of the C. paleea is examined in the spawning season, 
ova of different degrees of development are to be seen, but all consist- 
ing of the same essential parts. According to M. Vost, the following 
