78 THE AMERICAN ARBACIA 



b. Methods for Estimating Volume and Quantity 



Volwne. I . Direct measurement. Measure the diameters of many eggs of a sample, 

 convert to volume^, and take average. 



2. Diffraction method. Described by Lucke, Larrabee andHartHne, (1935). Used 

 by Luck^ et al. in subsequent papers; Korr (1937); Ballentine (1940b); et al. 



Number of eggs in a suspension. The main error in determining numbers of eggs is 

 caused by the jelly coat which varies in different batches of eggs, and may be 

 removed from some or all eggs by agitation, centrifuging, etc. The presence of the 

 jelly coat makes a difference of about 60 ju in the diameter of each egg, and it has 

 a volume of about 1,050,000 [i^. 



The following methods have been reviewed and appraised by Shapiro (1935 c). 



1. Haemocytometer. Used by Tang and Gerard (1932), Gerard and Ruben- 

 stein (1934) ; et al. 



2. Centrifuge, using haematocrit tubes. Used by Whitaker (1933a IV; 1935); 

 Clowes et al. (1936, etc.); Mazia (1937); criticized by Gerard and Rubenstein 



(1934)- 



3. Dilution method of Parpart, described by Shapiro (1935 c). This seems to 

 be the best method. Immerse quickly a capillary of i mm. bore into a uniform sus- 

 pension of experimental eggs, filling to a length of 8-10 cm. Lay the capillary on 

 its side under a binocular microscope, and count the number of eggs. Do this three 

 times. If the suspension is too thick, dilute to a suitable amount and allow for the 

 dilution factor in calculation of number of eggs in original suspension. Used by 

 Korr (1937); Ballentine (1940 b) et al. 



Number of cleaving eggs. Place the eggs at the desired stage in a weak formol solution, 

 0.04 to 0.5%, and count at leisure for percentage (Morgan, 1895 b and many 

 others) . 



c. Shape of Eggs 



Arbacia eggs are usually spherical when shed or removed from the 

 ovary. Sometimes they are aspherical due to crowding in the ovary, 

 especially late in the season; they become spherical on standing or on 

 fertilization (see also Goldforb, 1935 a). They become amoeboid with 

 ethyl urethane, urea, etc. See Amoeboid Eggs, Part. IV. 



They do not flatten by gravity (McCutcheon, Lucke, and Hartline, 

 1931; Cole, 1932; E. N. Harvey, 1933; E. B. Harvey, 1934). It was 

 claimed by Chambers (1921 a) that Arbacia eggs do flatten by gravity, 

 and by Vies (1926) that Paracentrotus lividus eggs flatten. Rothschild 

 and Barnes (1953) state that P. lividus eggs do not flatten by gravity. 



The egg oi Echinocardium cordatum (Kristineberg, Sweden) is peculiar 

 in being pear-shaped, but becomes spherical on fertilization (Gustaf- 

 son, 1945; Runnstrom, 1948). MacBride (1914b) describes it as "elUp- 

 soidal" at Millport, Scotland. The eggs of the sea urchins at Naples, 

 Sphaerechinus granulans, Psammechinus microtuberculatus, and Paracentrotus 

 lividus are much more aspherical than those of Arbacia punctulata at 

 Woods Hole (E. B. Harvey, 1933 a). 



* A good conversion table from diameters to volumes of spheres is given in the Chemical 

 Engineers' Handbook, 3rd edition, 1950, p. 34, 35; 2nd edition, 1941, p. 90, 91.. J. H. Perry, 

 editor. 



