EARL y BE VELOPMENT 



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.^. 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, Re/, p. 249). 



C. Tin-: KMHKVOXIC 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 Bohm, and Calberla ; Sharks by Riickert ; Te- 

 leostomes by Hoffman, Agassiz and Whitman, Kupffer, Bohm, and others. 



t These ' appear later to undergo karyokinesis, and are thereafter to be 

 regarded as supplemental merocytes (p. 195). 



