H. M. FocHs 291 



increasing the number of eggs fertilized in inter-specific hybridization, 

 namely, the preliminary treatment of the sperm with egg-extracts. 



As has already been mentioned, the sperm-suspension used in the 

 hybridization experiments of Table XXXII were not very concentrated. 

 Nevertheless, they were strong enough to show under the microscope 

 hales of spermatozoa collected round the eggs in the second parts of the 

 last two experiments. A comparison of the size of these halos is of 

 interest. In Exp. 2, halos were not visible round the eggs fertilized by 

 the water-suspension of sperm. They were present, however, both 

 round the eggs inseminated by the Stroiif/i/locetitrotus extract suspension 

 and by the Echinus extract suspension, and they were of equal size in 

 each. In Exp. 3 there was a slight zone of spermatozoa visible round 

 the eggs fertilized with the water-suspension, and strong, equally large 

 ones surrounding the eggs in the sperm-suspensions made up in the 

 two extracts. 



6. Comparison of the effects of Strongylocentrotus and Asterias egg- 

 extracts on the fertilizing power of Strongylocentrotus sperm. 



Von Dungem (1, p. -36) found that a substance can be obtained 

 from the eggs oi Asterias by breaking them up, which in small doses is 

 poisonous to the spermatozoa of Echinoids, but which does not affect 

 the sperm of the same species. The Echinoid spermatozoa could be 

 seen under the microscope to be killed by this substance. The latter is 

 given off by the eggs into the sea-water, and owing to its presence the 

 spermatozoa of Echinoids cannot enter the eggs of Asterias. 



This discovery appeared to me to be the more surprising as my 

 previous experiments had shown no such phenomena for the forms 

 experimented with. It was desirable, therefore, to repeat Von Dungern's 

 experiments with the Asteroid extract. This was earned out, using the 

 same strictly comparative method employed in the other experiments 

 described in this paper. 



The first trials were made with egg-extracts obtained in the usual 

 way. Definite equal amounts of Strongylocentrotus sperm-suspension 

 (the quantities are given in Table XXXIII) were added separately to 



TABLE XXXIII. (3.20.6.) 



StrongylocentTotus A stcrias 

 Water extract extract 



„ , „ , , , ( 3 drops sperm 8 68 



Exp. 1. Stronnylocentrotus dih { ^ „, ,„„ „ 



^ •"' ( 1 ec. sperm ... 62 100 



„ „ „ , , , \ 3 drops sperm 28 89 



txp. 2. btrongmocentrolus Ac { ^ ., , ,„, „ 



' ( 1 CO. sperm ... 84 100 



19-2 



