SEXUAL ACTIVITY OF MALE RABBITS 473 
NUMBER OF SPERM PER UNIT VOLUME 
A few investigators have made counts of the number of sperm 
per unit volume of semen recovered after the male has engaged in 
little or much sexual activity. Conspicuous among these is 
Lode, who: was the first to attempt a precise numerical deter- 
mination of the sperm content of mammalian semen. He pre- 
sents two observations on man; in the first case the number of 
spermatozoa per cubic millimeter decreased from 53,200 at the 
first copulation to 0 on the third copulation of that day. Ona 
second observation, the number decreased from 56,800 on the 
first to 19,400 on the second copulation. With a dog, the num- 
ber of sperm for the first ejaculation was 75,000 per cubic 
millimeter and fell to 3008 on the fourth ejaculation. With a 
second observation on the same dog, the numbers were 50,000 
on the first ejaculation and 29,000 on the fourth. Still a third 
count at a later period showed 56,840 on the first and no sperm 
on the fourth ejaculation. Mantegazza states (Stigler, ’14) that 
in semen of man obtained from the second ejaculation in one hour 
there were scarcely one-half as many sperm in the same volume 
as in semen from the first ejaculation. 
Iwanoff (07, p. 494) in writing of a stallion, says that the 
number of sperm cells decreased greatly during the third and 
fourth copulations in a day, but he does not give the numbers. 
Lewis (11) reports the number of sperm cells per cubie milli- 
meter in the semen from a draft stallion as 131,750 at the first 
service and 5840 on the ninth service made at the rate of one 
copulation daily for nine successive days. Another stallion 
showed 68,500 sperm cells per cu.mm. in semen from the third 
copulation made in two days and 23,000 per cu.mm. in the 
twentieth copulation made at the rate of two copulations daily. 
The counts reported from our own work do not represent ab- 
solute number of sperm per cubic millimeter, but a close ap- 
proximation to the same, based upon the assumption of 1.0 as 
the specific gravity of semen. As mentioned, volumetric methods 
with the very small quantities of this viscous fluid available 
are not suitable for quantitative work. In practice the whole 
