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 

 placed that they will be 

 sure 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 2 



FORMS OF SPERM CELLS (SPERMATOZOA) 



i 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) 



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. 



