54 REi PRODUCTION 



In Eudorina, which is very much like Pandorina, there 

 is much more difference in the size of the two kinds of 

 gametes. Some of the gametes are quite large and plump. 

 Others are small and spindle-shaped. The differences are 

 at once recognized under the microscope. One of the thin 

 cells unites with one of the plump ones. A small one 

 never unites with a small one, or a large one with another 

 large one. The result is the same as in conjugation. But 

 we call the process fertilization when one gamete is much 

 larger than the other. The small gamete is said to 

 fertilize the large one. This name expresses an early 

 guess as to the value which this union has. It suggests 

 that the small cell stimulates or nourishes the large one 

 for a better development. But we now have good reason 

 to think that the relation is a somewhat deeper one. Each 

 gamete makes a definite contribution to the individual 

 which results from the combination. 



3. Ovum and Sperm. What has just been described is 

 the beginning of the differences which we find in the 

 offspring (gametes) of all the higher plants and animals. 

 The larger of the gametes is an egg, the smaller is a 

 sperm, and it may be very small indeed as compared with 

 the egg. The difficulty now is not to tell them apart, but 

 to find anything in which they are alike. In appearance 

 they differ about as much as cells can. Both, however, 

 are single cells. In the egg there is a large amount of 

 nourishment stored by the parent. On the other hand, 

 the sperm is a cell with little more to it than a nucleus. 

 It carries no nourishment, but is usually exceedingly 

 active. All the higher plants and animals produce these 

 two kinds of cells. 



4. Eggs and Their Characteristics. An egg, as we have 

 seen, is a cell. It may have some very peculiar structures 

 and powers, but it must always be remembered as a cell. 

 The shell and the white of a hen's egg are not parts of the 

 egg in this sense. Scientifically speaking, the yolk is the 



