THE HYPOTHESIS OF SELECTIVE FERTILIZATION. 7/ 



may be led to study this important question by the same method. 



We made use of the eggs and sperm of the mollusk Cumingia. 

 The spermatozoa are quite large and can be readily seen with 

 moderate magnification. The eggs are small so that the entire 

 exposed surface can be watched closely. The eggs just laid 

 were put into a drop of water on a slide, and a cover slip added. 

 The cover slip was sufficiently supported so that the eggs were 

 not too much compressed. It was advantageous to apply pres- 

 sure because otherwise the spermatozoa may reach the &^o above 

 or below the horizon of observation ; such sperm are lost to 

 sight as a rule and their fate remains uncertain. Even with 

 moderate pressure the spermatozoon sometimes slips in between 

 the &^^ and the cover slip (above or below the horizon) and are 

 lost to sight. In such cases further observation is worthless. 



By means of a fine pointed pipette a small drop of water con- 

 taining not too many spermatozoa was introduced at the edge of 

 or under the cover slip. From the point of insertion the sper- 

 matozoa swim out in all directions and at some distance from the 

 starting point the path of a single spermatozoon could be easily 

 followed. The only way in which we were absolutely certain of 

 seeing the first spermatozoon that reached the t.^^ was to focus 

 on an ^^^ and wait until one came in contact with the &%^. 



It was seen that many spermatozoa swim past the ^^^ and 

 show no evidence of turning towards it, but those whose pre- 

 vious path was such that they ran into the jelly around the 

 €ggj bored into the jelly and often reached the surface of the 

 ^gg- Whether after a spermatozoon has entered the jelly it ever 

 turns towards the ^^^ (if it did not have this orientation at the 

 time of contact) is difficult to determine with certainty, but it is 

 certain that spermatozoa may bore through without turning 

 owards the egg. Some of the attached spermatozoa may show 

 alternate periods of rest and activity, and in consequence change 

 their position several times, and even end by entering the t^'g, 

 but there is no evidence that one position is more directed than 

 another. 



The successful spermatozoa are those that strike the Q.gg 

 " head-on," and bore directly towards the surface. When the 

 surface is reached the end of the spermatozoon appears to enter 



