416 GRACE MEDES 



grew slowl}^ and continuously for the five days. On the other 

 hand all of IV and all of C reached a maximum either at the 

 72 or the 96 hour period, indicating that this quality of rapid 

 development must be contributed by all of the eggs and all 

 of the sperm of those t\YO individuals. The combined effects of 

 these two forces is greater than that of either one alone, since 

 C IV reaches a lower point on the curve than any other culture 

 120 hours old. 



Some characteristic modifications in the skeletons were also 

 observable. For instance, all the cultures of A contained a few 

 individuals having four rods in one of the anal arms. This is 

 not an unusual variation in Arbacia plutei and is found in 

 many of the cultures. In this set, however, it occurred in 

 greater proportions than ordinarily happens, and usually on the 

 left side. Figures 147 to 150 are from AI, All, AIII and AIV 

 respectively. In each one four rods are present in this position 

 and in AI there is the rudiment of a fifth which might ulti- 

 mately have fused with the rest. 



In all the solutions of III, a large number of plutei had 

 the arms spread much more widely than is usual (fig. 151, 

 Bill, 48 hours). In a count of 100 specimens from each solution 

 taken after 48 hours, AIII possessed 79; Bill, 24; CIII, 81 

 and Dili, 97 with widely spread arms. In each of these solu- 

 tions there were also a few of the type represented in figure 

 152 (Bill, 48 hours). The arms and body-rods were about 

 the same length as in the normal but were almost parallel and 

 formed jight angles with the narrow transverse connectives 

 and the broad plate-like posterior portion of the body-rods. 

 The oral arms were spread much as in the normal specimens 

 and hence projected at the sides beyond the anal. Individuals 

 of this type averaged about one per cent of each of these cultures. 

 There were none like them to be found in any others of the series, 

 but in an entirely different solution fertilized with sperm from the 

 same male, a still greater percentage of skeletons of this type 

 was present. 



