THE BIOLOGY OF THE CELL SURFACE 



point of sperm-entry. Those farther removed may become 

 attached, lashing back and forth very vigorously, until 

 with farther penetration of the one successful spermatozoon 

 the "wave of negativity" reaches them and their movement 

 comes to a stand-still.^ 



The ectoplasmic changes in the egg of Nereis that take 

 place after sperm-attachment are also worthy of note. 

 Before insemination the ectoplasm of this egg is a broad 

 chambered structure. When eggs and spermatozoa are 

 mixed, -sperm-attachment rapidly ensues; then follows the 

 escape of material from the ectoplasm, which rapidly sets 

 in the sea-water as a transparent jelly. The ectoplasmic 

 chambers break down, leaving only strands that cross the 

 space between the inner part of the egg and the membrane. 

 Now comes a period of amoeboid changes in the egg:- it 

 shrinks, becomes markedly irregular in outline, its contents 

 become darker and the perivitelline space much reduced. 

 When this period is over, the t^^ assumes a clearer and more 

 rounded appearance; the perivitelline space widens again. 

 The delicate plasma-membrane beneath the vitelline mem- 

 brane is now easily visible. 



In this egg, thus, the ectoplasmic changes differ from 

 those observed in the eggs of Arbacia and of Echinarachnius . 

 In all three however the essential response to insemination 

 is the same, namely, an alteration in the ectoplasm. These 

 changes have significance for fertilization and experimental 

 parthenogenesis as we shall see. At present, another aspect 

 of them elicits our interest. 



By experiments with dilute sea-water, I was able to cor- 

 relate the structural changes at the surface of the eggs 

 named above with an alteration in the physical state of the 

 egg. I found, for example, that the egg of Echinarachnius 



1 Just, 1919. 



- Cf. Torrey, igoj on amoeboid changes in eggs of Corymorpha. 



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