STUDIES OF FERTILIZATION. 5 



thereof. The various inseminations are all so designated. Unit 

 sperm concentration would usually be called a "light insemina- 

 tion " and ten unit concentration would usually be regarded as 

 fairly " heavy." The absolute value of " unity " would be one 

 part of the dry sperm in about 70,000 parts of sea-water. 



There is not much difference in the relative fertility of the 

 crosses between the two species. In the records given it appears 

 that franciscanus eggs may give a small percentage of cross 

 fertilization at lower sperm concentrations than purpuratus eggs ; 

 but the purpuratus spermatozoon is much smaller than the fran- 

 ciscanus spermatozoon, so that it is probable that sperm suspen- 

 sions of purpuratus rated as of the same concentration as those of 

 franciscanus really contain a very much greater number of sperm- 

 atozoa, which would tend to explain the lower concentration for 

 minimum results in cross fertilization of franciscanus eggs. On 

 the other hand, the highest percentage of fertilizations recorded 

 in a cross (Table 4) concerns purpuratus eggs ; this may be due 

 to the much greater number of experiments with this cross than 

 with the reciprocal. 



The cross-fertilized purpuratus eggs appear to be more viable 

 than the cross-fertilized franciscanus eggs ; the latter do not in 

 my experience develop to the pluteus stage, while the former do 

 readily enough. The blastulae even of the cross-fertilized fran- 

 ciscanus eggs ofter appear abnormal, and the gastrulae very 

 commonly so. It is doubtful to what factor to ascribe this differ- 

 ence, whether to the large size of the cytoplasmic mass of the 

 franciscanus egg and small size of the purpuratus spermatozoon, 

 or to behavior of the chromosomes in the reciprocal crosses. 

 Material is on hand for investigating the latter possibility. 



Another difference noted in the two reciprocal crosses is that 

 the cross-fertilized purpuratus eggs commonly form as fine mem- 

 branes as the straight fertilized ones, whereas in a much higher 

 percentage of franciscanus eggs the membranes are either " tight," 

 in the sense that they do not stand out so far from the surface 

 of the egg, as in the straight fertilization, or lacking entirely. 

 Now the tight membrane and absence of membrane in fertilized 

 eggs are signs of poor condition or low vitality. This difference, 



