PHYSIOLOGY OF THE SPERMATOZOON 97 



b) Gravitation. Similarly animal spermatozoa are 

 not known to exhibit any definite reaction to gravi- 

 tation. The spermatozoa of marine forms are of some- 

 what greater specific gravity than the sea-water, hence 

 they tend to sink when at rest, or may be precipitated 

 by the centrifuge. 



c) Osmotic pressure. Spermatozoa of Nereis are 

 apparently more sensitive to increase than to decrease 

 of osmotic pressure. They are fairly active in 5 c.c. 

 of sea-water plus 2 . 5 c.c. of distilled water, but are 

 paralyzed in 5 c.c. of sea-water plus i c.c. of 2^M. NaCl 

 (Lillie, 1913). 



d) Temperature.- -Temperature affects the rate of 

 movement of spermatozoa; it is rather difficult to meas- 

 ure rate of movement directly, but in the case of Nereis 

 the aggregation reaction described on page 94, which 

 is a function of the activity, gives us a means of ready 

 observation : 



At 13 C. No aggregations form. 



At 1 5 C. Slight signs of aggregation in four minutes. 



At i8-i9 C. Aggregation in from two to four 

 minutes; much fewer in number than at higher temper- 

 atures. 



At 20.5 C. Numerous aggregations in one minute. 



At 23.5 C. Yet more numerous aggregations in 

 thirty seconds. 



At 26.5 C. No aggregations form until the tem- 

 perature falls to about 23. 



Thus in this case temperatures from 20 to 23.5 C. 

 are optimum. At 15 the movements of the sper- 

 matozoa are too slow to produce the aggregation re- 

 action, and at 26 . 5,^ although the movements are 



