EARLY DEVELOPMENT 187 
groups of fishes. In the forms which have thus far been 
studied * there have been few noteworthy variations from 
what appear the normal conditions of vertebrates. The 
sperm usually gains admission to the egg through a micro- 
pyle in the egg membranes which becomes formed imme- 
diately after the extrusion of the polar bodies. A sperm 
cell, invariably a single one, participates in the actual 
fertilization. This may occur directly by the formation of 
a single male pronucleus, as e.g. in Petromyzon, Teleosts ; 
while in the sharks, on the other hand, Riickert describes 
a multiple fertilization (polyspermy), where many male 
pronuclei} are formed, the one nearest in position fusing 
subsequently with the female pronucleus. An_ inter- 
mediate condition seems to be retained in the sturgeon, 
where several (six to nine) micropyles have been noted, 
although but a single one occurs in the kindred Ganoid, 
Lepidosteus (Mark, Ref. p. 249). 
Cc. THE EMBRYONIC DEVELOPMENT 
When the egg of a fish is deposited, it contains but the 
elements of a single cell. Its size and its enveloping 
membranes may vary widely, but its constituents are con- 
stant,— cytoplasm and nucleus. The size of the egg in 
different fishes varies with the amount of food material, 
or yolk, stored away in its cytoplasm; the enormous egg 
of the shark differs from the minute egg of the lamprey 
strikingly in this regard. But even in the minute lamprey 
egg there is a certain amount of yolk material present. 
In every egg there can usually be distinguished at sight 
*Lamprey by Kupffer and Béhm, and Calberla ; Sharks by Riickert ; Te- 
leostomes by Hoffman, Agassiz and Whitman, Kupffer, Bohm, and others. 
+ These appear later to undergo karyokinesis, and are thereafter to be 
regarded as supplemental merocytes (p. 195). 
