THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 127 



Hora as that of tlie rapidly changing tidal waters that sweep over the 

 beds where the Echini find their natnral habitat. Nevertheless a 

 number of the Plutei reared under these conditions have metamor- 

 phosed. 



II. MATERIAL. 



The experiments were commenced in the Laboratory of the Marine 

 Biological Association, Plymouth, in 1909, and continued through 1910 

 and 1911. Similar results were obtained from each series of experi- 

 ments. 



Echinus csculenhis and E. acutus are obtainable at Plymouth in fairly 

 large quantities by trawling in depths of about 25 fathoms. 



Echinus miliar is is a shore species. It is not so numerous as the 

 other species, and can only be collected at very low tides. 



The percentage of ripe Echini in a haul is small. The quantity 

 of material consumed was consequently large, and our thanks are due 

 to Dr. Allen and the staff of the Laboratory for the trouble taken to 

 provide us with a constant supply of material. 



To ensure successful fertilization it is essential that the sperm and 

 ova should be thoroughly mature. Partially ripe sperm and ova are 

 capable of fertilizing, and of being fertilized, but the larvse of such 

 a union do not develop normally, and seldom reach a late stage, 

 although they may remain alive for many days. 



It might be assumed that the ideal condition would be to use Echini 

 on the point of discharging their genital products, and such under 

 normal conditions would be correct, but we have frequently noticed 

 that under irritation or when in an unhealthy condition the animals 

 discharge their sperm and ova. In a few instances we have observed 

 them discharging in vast quantities when in a moribund condition, the 

 urchins dying afterwards within a few hours. It is almost unneces- 

 sary to add that, when eggs or sperm, shed in this manner, are brought 

 together, fertilization either fails to take place, or the resulting Plutei 

 are abnormal and unhealthy. 



If a ripe ovary is gently shaken in water, the ova will float away 

 without any or but a small quantity of follicular tissue. Rough shak- 

 ing should be avoided, as by it many partly ripe and immature eggs 

 will be detached. The ovary from which the greatest number of eggs 

 is obtained with the least agitation, and the least admixture of the 

 tissue, will generally be found to give the best result. Under the 

 -microscope the nuclei of such eggs will not be visible. 



It is more difficult to judge the ripeness of the spermatozoa, as they 

 are motile some time before they are actually mature. The mature 



