134 THE AMERICAN ARBACIA 



like the unfertilized ones, but in not so well-defined layers (Plate XII, 

 Photograph 15; cf. with Plate VII, Photograph i). As is well known, 

 they become more viscous on fertilization (Heilbrunn, 19 15, 1920a, 

 1928, p. 264; E. B. Harvey, 1932, 1933 b; Goldforb, 1935 b). Except 

 for a short period immediately after fertilization, they break apart 

 into white and red halves like the unfertilized eggs; these are usually 

 of the same relative size as the unfertilized halves, the white half a 

 little larger than the red, except if centrifuged just before cleavage 

 when the white half is quite small. If centrifuged one or two minutes 

 after fertilization, the eggs break into very small pieces (E. B. Harvey, 

 1933b, 1940b) (Plate XII, Photograph 14). 



The fertilization membrane must be removed by vigorous shaking 

 just after it is formed, in order to allow the eggs to elongate and break. 

 If centrifuged during the monaster stage, 6-20 minutes after fertiliza- 

 tion, the eggs form long streamers which retract into spheres on re- 

 moval from the centrifuge. This is particularly noticeable with low 

 forces and readily observed with the centrifuge microscope (E. B. 

 Harvey, 1933b ) (Plate XII, Photograph 17). If centrifuged slowly as 

 the fertilization membrane is forming, this may be greatly stretched 

 to a length of 128 // from a diameter of 80 //, and the egg may break 

 into two parts inside the stretched membrane. If the force is removed, 

 the two parts may coalesce or may remain and develop separately; 

 in the latter case, the female nucleus may be in the white half and the 

 male nucleus in the red half; the red sphere with the male nucleus 

 usually develops better than the white sphere with the female nucleus. 



When eggs are centrifuged after fertilization, the hyaline layer is 

 thrown off as a ring or crescent into the perivitelline space. This is 

 much more striking in eggs with a large perivitelline space, e.g., 

 Psammechinus microtuberculatus, and is observed better in Arbacia punctul- 

 ata if the fertihzation membrane is broken (E. B. Harvey, 1934) 

 (Plate XVI, Photograph 6). The hyahne layer is later replaced, and 

 if the fertilization membrane remains intact, the egg develops normally. 



