io6 



THE NEXT GENERATION 



launched into life. This trouble comes from the fact that 

 every generation of germ cells for every kind of animal has 

 to meet three conditions : 



1 . Two separate cells — one each from two separate 

 animals — must unite to form each member of the next 

 generation. 



2. To live, they must be prevented from getting dry. 



3. They must be so 



11 



>.(/- 



Forms of Sperm Cells (Spermatozoa) 



1 and 2, immature and mature spermatozoa of 

 snail; 3, of bird; 4, of man; 5, of salamander; 

 6, of Ascaris ; 7, of crayfish. (Enormously mag- 

 nified. Not drawn on scale.) (From Thomson) 



placed that they will be 

 sine to find each other. 

 Fish meet these con- 

 ditions as follows : 



The female fish, as it 

 swims about, first de- 

 posits a mass of germ 

 cells, each one of which 

 is an egg — an ovum. 

 Later the male fish, also 

 swimming that way, de- 

 posits on the eggs a clear-looking substance which seems to 

 resemble nothing so much as the white of an egg. This sub- 

 stance really contains millions of germ cells. Each one is 

 called a sperm, and each is capable of fertilizing one of 

 the eggs. 1 



It is a fairly easy matter to discover separate eggs in the 

 vast numbers which a fish lays at one time, for they float 

 about, looking like bits of jelly bunched together. It is 

 otherwise with the sperm cells. All that the eye sees is a 

 thickish liquid, but under the microscope, in the liquid it- 

 self, we see numberless darting objects that are in constant 



1 A mature germ cell is often called a gamete. When two germ cells, 

 or gametes, unite they form a fertilized cell called a zygote. 



