Sexual Cells on the Development of Uchinoids. 359* 



of ten observations, the stale $ fresh J larvae were diminished 6 '9 

 per cent. These values, being means of a fairly large number of 

 observations, may be regarded as trustworthy within certain limits, and 

 will, I think, be held sufficient to justify the rule above laid down. 



Confirmation of the conclusion that larvae derived from stale ova and 

 fresh sperm are smaller than the normal was obtained from quite another 

 source. Thus in one case, it was found that the hybrid larvae obtained 

 on crossing the twenty -four hours stale ova of Sphcerechinus granulans' 

 with fresh Strong ylocentrotus sperm were 5*4 per cent, smaller than those 

 from the cross of the fresh ova and sperm, whilst those from some of 

 the same stock of ova after keeping an additional nine hours, and then 

 crossing with fresh sperm, were 9*3 per cent, smaller. Again, in another 

 experiment, larvae from twenty-four hours stale Sjjhcerechinus ova crossed 

 with fresh Strong ylocentrotus sperm, were 3*8 per cent, smaller than those 

 from the fresh ova and sperm. None of the repeated attempts at 

 obtaining crosses with stale ova and stale sperm succeeded, and in the 

 only case in which, fresh ova were crossed by stale sperm, the cross of 

 fresh ova and fresh sperm failed, and so prevented any comparison 

 being made. 



In a previous part of the paper it was shown that the proportion of 

 blastulae obtained with both sexual products stale was nearly as great 

 as that with only one of them stale, and a similar relationship was found 

 to extend to the proportion of larvae. Thus, excluding the series of 

 observations made after forty-five hours, in which only 0*4 per cent, 

 or less of the ova arrived at the larval stage, and including only those 

 series in which all three of the fertilisations were attempted, it was 

 found that on an average stale ova with stale sperm yielded 43*5 per 

 cent, of larvae, stale ova with fresh sperm 55*5 per cent., and fresh ova 

 with stale sperm 49 "0 per cent. In this case, therefore, as in that of 

 the blastulae, the stale ova with stale sperm yielded the least propor- 

 tion, and the stale ova with fresh sperm the greatest, but the extreme 

 limits of variation are only comparatively slight. 



It may perhaps be asked whether this somewhat curious result as to 

 the effect of staleness of one or other of the sexual products on the 

 size of the larvae is at all likely to be a factor of any importance 

 under natural conditions. To me it seems that in at least one respect 

 its value may be considerable, viz., it may be a very potent cause of 

 variation. As has been already stated, it seems probable that the 

 condition of relative staleness of the sexual products at the time of 

 fertilisation may very frequently occur in several phyla of the animal 

 kingdom, and hence it is by no means improbable that the average 

 variability of each generation may be increased considerably by this 

 means. As has been shown elsewhere,* the mean probable error of 

 variation in the body length of these Strongylocentrotus larvae is 6*1 per 

 * ' Phil. Trans.,' B, 1895, p. 615. 



