'360 Effect of Stale/less of Sexual Cells on Development. 



cent. If, then, with a particular sea-urchin a third of the ova did not 

 undergo fertilisation till some twenty hours stale, whilst another third 

 underwent fertilisation by twenty hours stale spermatozoa, and only the 

 remaining third underwent fertilisation at once by fresh spermatozoa, 

 then the probable error of variation of all of the larva? so arising would 

 roughly speaking be doubled. Of course this is an extreme instance, 

 which could never occur in the case of Echinoids, but might easily 

 occur in the case of, for instance, man. Whether the variations so 

 produced would be in any degree transmissible by inheritance, is quite 

 another point, but supposing it to be only the size of the offspring 

 which is thus influenced, it may be merely a question of varying 

 degrees of nutrition, and so be directly transmissible. In any case, 

 whether inheritable or not, variation of itself may be in many cases 

 of value, as it may give a better chance to natural selection and other 

 agencies of picking out those individuals more adapted to their en- 

 vironment, and rejecting those less adapted. Thus, if all the individuals 

 are nearly alike the evolutionary process must needs be exceedingly 

 slow. 



Finally, these results are of interest in that they prove the inequality 

 of the sex cells. The diminution in the size of larvse obtained from 

 stale ova and fresh sperm may perhaps be looked upon as one of 

 diminished nutrition, the result of the staleness of the yolk, but it is 

 difficult to imagine why the staleness of the spermatozoon used to 

 fertilise an ovuni should produce a larva larger than the normal, unless 

 one holds that the part played by the sex cells differs in some essential 

 particular. 



Summary. 



The following are the chief conclusions arrived at in this paper : — 



(1) If the ova and sperm of the Echinoid St'rongt/loeentrotus lividus be 

 kept for various times in sea water before fertilisation, then for about 

 the first twenty to twenty-seven hours the number of normal blastula? 

 formed diminishes only about 1 per cent, per hour. After this ab- 

 normal development sets in rapidly, so that generally after a further 

 nine hours or so, no blastulse at all are obtained. The rate of falling 

 off in the number of normal blastulse may increase to as much as 18'9 

 per cent, per hour. 



(2) If ova not more than twenty-seven hours stale be fertilised 

 with equally stale sperm, practically as many blastuke are obtained 

 as when stale ova are fertilised by fresh sperm, or fresh ova by stale 

 sperm. After twenty-seven hours, however, the number of blastula? 

 obtained with both products stale falls off" more rapidly. 



(3) Larvas obtained from stale ova and stale sperm are of practically 

 the same size as those from fresh sexual products, but those from fresh 

 ova and stale sperm are distinctly larger than the normal, whilst those 

 .from stale ova and fresh sperm are distinctly smaller. 



