474 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [20] 



"5. Division of the spindle into two halves, one reniaininj? in the egg, 

 the other in the i)romiuence ; tlie prominence becoming at the same time 

 nearly constricted ott" from the egg as a polar cell. 



" 0. The formation of a second polar cell in the same manner as the 

 first, part of the spindle still remaining in the egg. 



"7. Conversion of the part of the spindle remaining in the egg into 

 a nucleus — the female i)ronucleus. 



" 8. Transportation of the female i)ronucleus towards the center of 

 the Ggg. 



"9. Entrance of a single spermatozoon into the egg. 



"10. Conversion of the head of the spermatozoon into a nucleus — the 

 male pronucleus. 



" 11. Appearance of radial striiE round the male pronucleus, which 

 gradually travels towards the female pronucleus. 



"12. Fusion of male and female pronuclei to form the lirst segmenta- 

 tion nucleus." 



The foregoing account is essentially the sequence of events as observed 

 by Fol in Asterias ijlaclalu. This series of events evidently does not 

 hold for all forms. Thus, in Ilinidinea, MoUiisca, and Nematoldeahnpreg- 

 uation takes jjlace normallj'^ before the extrusion of the polar bodies is 

 completed (Balfour), so that the event which stands as ninth in the pre- 

 ceding scheme would actually stand first, as in the case of the egg of 

 Ostrea, where no disposition is manifested to extrude polar globules 

 until the ova are brought into contact with the spermatozoa. In the 

 case of the lamprey, Kupffer and Benecke have shown that only one 

 spermatozoon enters the egg, but that others pass through the vitelline 

 membrane, and are taken into a peculiar protoplasmic protuberance of 

 the ovum which appears after impregnation. In ova of Ostrea virginica 

 which have been killed and hardened in osniic acid the ])ellucid tract 

 which penetrates the egg for some distance in the vicinity of the polar 

 globules i)robably represents the axis of the amphiaster formed at the 

 time of the development of those bodies. I am well assured of the 

 fact that no tendency toward a reorganization of the centrally ])laced 

 nucleus of the mature egg of the oyster is ever manifested until it is 

 brought into contact with si)ermatozoa. The egg of the osseous fish is 

 scarcely referrible to either of the foregoing categories; it has lost the 

 germinal vesicle as a central structure before it leaves tlie i)arent folli- 

 cle, and coincident with the development of the germinal pellicle or 

 germinal protoplasm which covers the yelk or dentoplasm, its substance 

 lias probably been mostly transferred to that layer. As we now know 

 that the germ disk is formed at the time of imi)regnation or independ- 

 ently of it, it is to be supposed that in this process the germinative 

 vesicle or its remains may not imi)robably undergo a com])lex metamor- 

 phosis. Although I have not yet met with anything that I could regard 

 as undoubted i)olar cells, a minute prominence occurs on the disk of 

 the cod which may be regarded as such, or as an apparatus for the 



