PHYSIOLOGY OF THE SPERMATOZOON 97 



b) Gravitation. — Similarly animal spermatozoa arc 

 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. Y^et 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 



