HOW REPRODUCING CELLS GET TOGETHER 11 
spermatozoon, which lacks the sex-determining chromosome, 
fertilizes the egg, the offspring is a male. We do not know of 
any way in which we can influence one of the two spermatozoa 
in preference to the other to fertilize the egg. Therefore we 
cannot control the sex. By the law of chance the males and 
females are about equal in number. 
1. What determines the sex of the offspring? 
2. Why have we not been able to control the sex of offspring ? 
Twins. — Most animals have a characteristic number of 
young produced at one time. How many are produced de¬ 
pends chiefly on the number of eggs which mature at one time. 
Some fish have millions; cats and dogs usually have three 
or four to eight or ten; sheep commonly have two; horses 
and human beings one. Human twins are commonly caused 
by two eggs maturing at the same time. Such twins may be 
the same sex or brother and sister and are no more alike than 
are other sibs. But sometimes a fertilized egg, early in its 
growth, divides into two parts which separate, each forming 
a separate individual. Twins so formed are called identical 
or one-egg twins. Since they have the same chromosomes 
they are in all instances the same sex and are so alike that 
even their parents can hardly distinguish one from the other. 
Why are some twins so alike as to be hardly distinguishable from 
each other while other twins have only a sib resemblance ? 
How Reproducing Cells Get Together. — In Flowers. 
The pollen grain (commonly a yellow dust in flowers) is a 
minute body which contains the sperm cell. The egg cell is 
within the pistil , the part which becomes the pod when the 
eggs develop into seeds. From the pollen which has been 
brought to the tip of the pistil, a long tube grows down 
through the pistil until it reaches the ovule (little egg), where 
the half nucleus of the sperm cell joins that of the egg cell. 
