556 



Phylum Echinodermata 



granules renders the living egg quite opaque; cytological details during 

 development may be seen only vaguely in most respects. 



When fertilizing eggs, Just dilutes one drop of "dry" sperm with 10 cc. 

 of seawater, and adds two drops of this suspension to the eggs in 250 cc. 

 of seawater (1928). Dr. Ethel B. Harvey fertilizes eggs by dipping a 

 toothpick in the "dry" sperm and stirring it in the egg-water. The exact 

 concentration of the sperm is relatively unimportant, since Arbacia eggs 

 are not easily subject to polyspermy. "Dry" sperm, obtained as de- 

 scribed above, are in good condition for 6 to 8 hours, but it is the usual 

 practice to obtain sperm from a freshly opened male each time eggs are 

 fertilized. 



As is the case with the eggs of most species, crowding should be 

 avoided, and the water changed from time to time. However, if the 

 experimental conditions require it, Arbacia eggs withstand the deleterious 

 effects of crowding to an unusual degree. 



The usual criteria which indicate that an egg-set is developing nor- 

 mally are as follows: (1) Distinct fertilization membranes are elevated 

 within about two minutes; if some of them are wrinkled or attached 

 to the egg at several points the set is in poor condition. (2) Practically 

 all of the eggs cleave. From the time when about 20% of them have 

 divided until about 80% have done so requires not more than 5 minutes 

 at 20 C; if this time is considerably longer, the egg-set is subnormal. 

 (3) First-cleavage blastomeres are shaped like half-spheres which are 

 flattened against each other and have smooth contours; if some of the 

 blastomeres are rounded like spheres, or show occasional protuberances, 

 the set is not normal. (4) Top-swimming plutei are produced in prac- 

 tically all cases; if numbers of embryos are bottom-swimmers the set is 

 in poor condition. 



Various workers {e.g., Goldfarb, 1929, and Just, 1928) have demon- 

 strated that the egg-sets of different Arbacia females differ in various 

 respects. Fry (1936) has analyzed variation in the time when different 

 egg-sets cleave, under various laboratory conditions, and has shown 

 that the time when any mitotic phase is at its "peak" is a constant 

 proportion of the cleavage time, no matter when cleavage occurs. This 

 is of practical value in relating cytological changes to physiological ones 

 in these eggs. If samples of an egg-set are being subjected to a given 

 experimentaltreatment at intervals after fertilization, the cytological con- 

 dition at each interval is easily calculated if the cleavage time is known. 



Harvey (1932) summarizes previous studies of the effects of tempera- 

 ture upon the time of cleavage. Fry (1936) reports the time schedules 

 of the detailed mitotic phases. Dr. Ethel B. Harvey has found that if 

 the eggs are kept at about 8° C. they retain their capacity to be fertilized 

 for a week or so. 



