CHAPTER 21 



Development of centrifuged eggs and fractions 



a. Spherical Centrifuged Eggs. Plate VIII 



If the eggs of Arbacia are centrifuged with a low force (e.g., 3,000 times 

 g for two minutes), they stratify but do not elongate or break apart; 

 they remain spherical. When these spherical eggs are fertilized, the 

 cleavages are quite normal (E. B. Harvey, 1932, 1940c, 195 1). The 

 first cleavage usually (but not always) comes through the axis estab- 

 lished by centrifuging, at right angles to the stratification; the second 

 cleavage is at right angles to the first and parallel to the stratification, 

 and the third at right angles to the first two and to the stratification. 

 The micromeres may come off anywhere without regard to the strati- 

 fication, but approximately opposite the micropyle, which marks the 

 original axis of the egg (Plate VIII, Photographs 15, 16). Plutei 

 developed from these eggs, normal in every respect except that the 

 pigment was localized somewhere, in any part of the larva. These facts 

 were established by the early work of Lyon (1906 a, 1907) and Morgan 

 and Lyon (1907). Further studies with regard to polarity and develop- 

 ment were made by Morgan (1908, 1909), Morgan and Spooner 

 (1909), and Spooner (191 1), and I can confirm them. A general sum- 

 mary of cleavage in these eggs and in egg fractions will be found in 

 the paper of E. B. Harvey (1951). A resume and able discussion of 

 polarity and centrifuged eggs is given in Morgan's Experimental Embry- 

 ology, 1927, p. 496-500. 



b. Elongate Centrifuged Eggs. Plate VIII 



As mentioned, the elongate eggs when unfertilized resume their spher- 

 ical shape within a few hours. If, however, they are fertilized immedi- 

 ately in sea water they retain their elongate or dumbbell shape (E. B. 

 Harvey, 1932, 1940c, 1951). The fertilization membrane follows the 

 contour of the egg, but there is usually left a slight bulge of the mem- 

 brane at the centripetal pole, owing to a slight shrinking of the egg 

 immediately after the fertihzation membrane has formed. There must 

 be a gelation or setting of the protoplasm following fertilization, for 



