266 



FERTILIZATION 



CERMINAL VESICLE 



MICROPYLAR CANAL 



I V L LOUS LAYE R 



I ZONARAOATA 



-y^ 



.^dihjiuiM 



Fig. 134. Micropyle and egg membranes of certain fishes. (A) Micropyle, egg mem- 

 branes, and germinal vesicle in Lepidostcus. (Modified from three figures drawn by 

 E. L. Mark, Bull. Mus. Comp. Zool. at Harvard College, 19: No. 1.) (B-F) Micropyle 

 and egg membranes of the cyclostome, Petromyzon planeri. (Slightly modified from 

 Calberia, Zeit. Wiss. Zool., 30.) (B) Mature, unfertilized egg. (C) Sperm passes 

 through the micropyle and enters the protoplasmic strand, P.S. (D) Higher power view 

 of sperm in protoplasmic strand; also observe that the egg is shrinking away from the 

 egg membrane, forming the perivitelline space. (E, F) Egg contracts away from the 

 egg membrane, leaving the egg free to revolve within the membrane. 



sperm. Development is not normal and does not go beyond the larval con- 

 dition. Parthenogenetic merogony is the development of non-nucleate parts 

 of the egg which have been artificially activated. Artificial activation of non- 

 nucleate parts of the egg of the sea urchin, Arbacia, is possible by immersion 

 of these parts of the egg for 10 to 20 minutes in sea water, concentrated to 

 about one half of the original volume, or by the addition of sodium chloride 

 to sea water to bring it to a similar hypertonicity (Harvey, '36, '38, '51). 



