ASYMMETRY IN THE STARFISH. 113 



the minimum sperm concentration in order to avoid polyspermy. 

 The standard procedure adopted and used in all experiments was 

 as follows: Five drops of solid sperm, just as it exudes from the 

 genital pores, was mixed with 100 cc. of sea-water; and then ten 

 drops of this sperm suspension was added to a finger-bowl con- 

 taining a single layer of eggs and 100 cc. of sea-water. Eggs must 

 stand for about an hour in sea-water before insemination is 

 attempted. 



CLEAVAGE, BLASTULA, AND GASTRULA. 



Cleavage begins after about two hours and is entirely similar to 

 that of other asteroids previously described. To get the best 

 results it is necessary to keep the cultures in an unheated room in 

 which the temperature ranges only a degree or so above or below 

 15 C. Especially is it important that the embryos should not be 

 warmed; a few degrees of lower temperature may be beneficial. 

 The cultures should also be shaded in such a way that no direct 

 sunlight reaches them. When eggs are reared under these pre- 

 cautions they reach a blastula stage in a little under or over 18 

 hours, and hatch out as swimmers in about 24 hours. The great 

 majority of the blastulae are practically spherical in form. Even 

 in the best of cultures there are always a few abnormal larva? that 

 are wrinkled or nearly solid, but in spite of their deformity 

 apparently more active than normal larvae. It has not been 

 determined whether these abnormal larvae are the product of over- 

 ripe eggs, of polyspermy, or of parthenogenesis. It is a simple 

 matter to rid the cultures of all such abnormal larvae at an early 

 stage, and thus to make conditions better for the normals. 



Gastrulation begins a few hours after the larvae have hatched, 

 and is in no way different from that described for other asteroids. 

 The various steps in the process are shown in Figures 1-6. It is 

 to be especially noted that gastrulae reared in this fashion are 

 quite free from any apical thickening such as Heath (1918) 

 described for this species and which he considered homologous 

 with the apical plate of the enteropneustan larva. Only sub- 

 normal larvae show this structure, as was pointed out in a previous 

 paper (Newman, 1922), but this becomes especially obvious when 

 improved methods of obtaining eggs are used. 



