STS ay EST, 
= a 
fend W 
1886, ] Embryology. 987 
The great majority of species of both fresh and salt water fishes, 
which have heavy, subsident ova containing oil, have their eggs 
provided with thick, heavy membranes, which are adherent to 
each other or to foreign bodies, or to both. Furthermore, their 
€gg-membranes are usually adhesive, with the oil-drops scattered 
beneath the surface of the vitellus, or aggregated in a flat, dis- 
coidal group beneath or alongside of the germinal disk, and not 
very transparent. The whole egg is also usually more or less 
colored or granular. The egg-membranes of those species which 
have buoyant ova are, on the other hand, characteristically thin 
and delicate, so that it is difficult, if not impossible, to make out 
the presence of pore canals, while the whole egg is, as a rule, re- 
markably transparent. ; 
These characteristics seem to show that the buoyant ovum is a 
very well-defined and specialized type, which has been developed 
in the course of the struggle for existence to serve a very useful 
Purpose in insuring the protection and survival of the embryo dur- 
ing the hatching period. 
_ There are fresh-water forms, also, which have buoyant ova, as 
in the case of Macropodus venustus, in which the proportional vol- 
ume of the oil-drop is greater than in any other known type. e 
oil in this case when liberated at once floats at the surface, as does 
e egg when entire, while the plasma of the germ and vitellus at 
once sinks. This fact, it seems to me, finally and conclusively 
Proves that the pelagic or buoyant habit of many fish ova is due 
to the presence of oil aggregated, as a rule, at the vegetative pole 
of the vitellus in the form of a single drop. The other conditions 
are (1) that the egg be free and not adhesive, with a thin mem- 
brane, and (2) that it be immersed in water having a greater density 
n 1.014. The one notable exception to the last part of this 
eneral statement, viz., Macropodus,} it seems to me, serves to 
show that the presence of oil is very important, and may excep- 
Bas be the sole cause of the buoyancy of the egg.—/ohn A. 
er, 
observing the development of the common mackerel, Scomber 
|For an “pportanity to study the development of this form, I am indebted to my 
hi. P. Seal. : 
