begins. If the sperm suspension is too concen- 

 trated, many spermatozoa enter one egg and 

 cause polyspermy, a condition which may inter- 

 fere with normal development of the egg. 



The spermatozoon which succeeds in pene- 

 trating the egg's surface midergoes great changes. 

 Its acrosome region becomes swollen and dis- 

 rupted and the tail loses its motility; the head 

 gradually penetrates the egg membrane as the 

 sperm moves deeper into the cytoplasm. At the 

 same time the fertilized egg contracts and assumes 

 a globular shape if it was not round before; the 

 cytoplasm becomes so dense that the germinal 

 vesicle is no longer visible under the layer of yolk 

 granules. A few seconds after the sperm head 

 touches the egg's surface a thin, transparent 

 fertilization membrane is elevated and imder the 

 hght microscope appears to be homogeneous. 

 This membrane apparently is formed from the 

 pre-existing vitelline membrane and is under- 

 lined by a layer of subcortical particles (fig. 318). 

 The two layers are optically separated. It is 

 generally accepted (Runnstrom, 1952) that in 

 Arhacia and many other species the fertilization 

 membrane originates from the vitelline membrane 

 because it fails to form after the vitelline mem- 

 brane has been removed with potassium chloride, 

 trypsin, or urea. No experimental work of this 

 type has been done on oyster eggs. 



AGING OF EGGS AND SPERM 



The longevity of eggs of marine invertebrates, 

 i.e., their ability to form fertilization membrane 

 and undergo cleavage, was observed in the sea 

 urchin {Arhacia) and in other common species 

 (Harvey, 1956). Oyster eggs also undergo aging 

 changes and lose their ability to be fertilized. 

 This has been demonstrated in a number of tests 

 made in the Bureau's shellfish laboratory at Woods 

 Hole. Because of wide individual variability in 

 fertilization capacities only one female and one 

 male were used in each series of tests. The 

 following technique was used : Suspension of egga 

 was made by shaking 0.5 g. of ripe ovary tissue 

 in about 200 ml. of sea water: eggs released by 

 this action were permitted to settle on the bottom 

 and the supernatant water was decanted; the 

 remaining eggs were rinsed twice in sea water and 

 transferred to a beaker filled with 500 ml. of filtered 

 sea water. The beaker was kept half submerged 

 in running sea water to prevent heating to room 

 temperature. Samples of eggs were taken for 

 fertilization every hour during the first 4 hours, 

 then at 2-hour intervals for the next 6 hours, and 

 finally one sample was taken each time after 12 

 and 24 hours. Eggs were collected at random 

 from the bottom of the beaker and placed in a 

 finger bowl in 100 ml. of filtered sea water. To 

 fertilize them 0.5 ml. of dilute stock suspension 

 of sperm was used; the water was gently stirred 



• % 



S.p. (> 







Microns 



8 



Figure 318. — Photomicrograph of a portion of fertihzed egg of C. virginka shorly after the attachment of sperm. Fer- 

 tilization membrane (f.m.) (outside layer) is underlined by the vitelline membrane (v.m.) with a dense row of subcor- 

 tical particles (s.p.). Live preparation. Oil immersion phase contrast lens. 



EGG, SPERM, FERTILIZATION, AND CLEAVAGE 



343 



