290 Studies in the Physiol (xjn of Fertilization 



that tl;e percentage of X eggs fertilized by Y speriii would be increased 

 to an equal extent by previous treatment of this sperm (1) by X egg- 

 extract, and (2) by an equal concentration of Y egg-extract. That such 

 is the case is proved by the following three experiments in which the 

 cross Echinus microtuherculatus $ x Strong tjlocentrotus lividus </ was 

 tested, the actions of egg-extracts of these two species being compared. 



Although Strong ijlocentrotns sperm of considerably greater concen- 

 tration is re(iuired to effect hybridization with Echinus eggs, than to 

 fertilize eggs of the same species, it is by no means necessary to make 

 use of milky suspensions. Indeed, if such are employed it often happens 

 that 100"/,^ of the Echinus eggs are fertilized, which is (jf course quite 

 useless for these comparative experiments. 



In each experiment 5 cc. of Strong ijloceiitrdtus sperm-suspension was 

 pipetted into each of three dishes containing (1) 5 cc. water, (2) 5 cc. 

 Strongylocentrotus egg-extract, (3) 5 cc. of an approximately equal con- 

 centration of Echinus egg-extract. Each was then poured on to separate 

 approximately equal quantities of Echinus eggs. Each experiment was 

 made double, weaker and stronger sperm-suspensions being used. 

 Table XXXII shows the results of the experiments. 



TABLE XXXII. (3.1.6.) 



Strongylo- 

 ccntrottut Echinus 

 Water extract extract 



„ , _ ,. , ,„ , ( Weaker sperm ... '2-2 32 31 



^ ' ■'■' \ Stronger sperm... .58 71 70 



^ , „ ,. , ( Weaker sperm ... 12 40 39 



txp. 2. Echttius A/StronqiiloceiUrottis n < „ 



^ ' ••' ( Stronger sperm... 0.5 08 70 



_,„_,. , ,„ , ( Weaker sperm ... 40 70 70 



Exp. 3. Echinus A/Strunriiilocentrotus p <„ 



' ■• ' I Stronger sperm... 72 71 73 



In all the experiments except the second part of Exp. 3, both 

 extracts raised the percentages of eggs hybridized by an equal amount, 

 allowing for 2°/^ experimental error. That there was no raising of the 

 percentages in the second part of Exp. 3 is explained by the fact that a 

 sample of the Echinus eggs A, fertilized with Echinus sperm in plain 

 water, showed only 74 7o of segmenting eggs. Evidently there was a 

 considerable number of unfertilizable eggs present, and the maximum 

 possible percentage had been touched on in the fertilization by untreated 

 sperm in the second part of Exp. 3. 



Although the experiments described in the last five sections have 

 shown that the hypothesis which they were made to test is unten- 

 able, the results given in Table XXXII indicate a valuable method of 



