74 THE AMERICAN ARBACIA 



free sea water, though the sperm are motile (Loeb, 1915a; E. B. H., 

 u.npub. ; see also Monroy, 1949 for Ps. microtuberculatus) . The papillae 

 do not form on immature eggs which have been in hypertonic sea 

 water for 20 minutes and then returned to sea water, a method of 

 producing parthenogenesis of mature eggs (E. B. Harvey, 1938a for 

 Ps. microtuberculatus). Sugiyama states that the papillae do not form 

 if the sperm have been treated with ultraviolet light, but this is not 

 true for Arbacia (E. B. H., unpub.). 



d. Centrifuging and Gravity 



On centrifuging, about 10,000 x g for 3 minutes, the mature unfer- 

 tilized egg is well stratified. The immature egg is only partially strati- 

 fied, with a small oil cap at the centripetal pole and pigment at the 

 centrifugal pole and above this the yolk but sometimes not well separ- 

 ated from the pigment (Photograph 13). The clear layer is not well 

 defined, and there is no mitochondrial layer. After centrifuging the 

 mitochondria remain as a thick layer around the wall of the germinal 

 vesicle as shown in Photograph 18 (not centrifuged). The egg does not 

 elongate nearly as much as the mature egg, and does not break apart 

 with the forces which are suflficient to break the mature egg into halves. 

 The immature egg is thus shown to be much more viscuous than the 

 mature egg. This has been noted by previous observers (Heilbrunn, 

 1928, p. 278; 1952, p. 85; Goldforb, 1935b). 



The germinal vesicle, in centrifuged eggs, lies at the centripetal pole 

 under the oil, and the nucleolus is heavy and always lies at the bottom 

 of the germinal vesicle (Photograph 13). Gray (1927 b) has made some 

 interesting studies on the effect of gravity on the nucleolus of the 

 Echinus esculentus egg. He found that the nucleolus descends by gravity 

 at the rate of 0.4 \j. per second (1.5 mm. per hour). The nucleolus in 

 the eggs of some species can be driven by strong centrifugal force, with 

 an air turbine, right through the germinal vesicle wall. Such is the 

 case with Strongylocentrotus drobachiensis (E. B. H., unpub.). 



e. Respiration 



The rate of respiration is less in immature than in mature eggs ac- 

 cording to Boell, Chambers, Glancy, and Stern (1940). Borei (1948) 

 found it slightly higher in Psammechinus miliaris, and Lindahl and Holter 

 (1941) in Paracentrotus lividus. 



