256 BIOLOGY 



sometimes rounded, but more commonly elongated. A careful 

 examination of this head shows that it contains an equivalent 

 of the two chromosomes originally present in the matured egg. 

 The spermatozoan head is therefore really a nucleus. Just 

 back of the head is a short piece known as the middle piece, 

 which contains a centrosome. This is the smallest part of the 

 sperm > The third part of the sperm is the tail, which is usually 

 rather long and motile, and whose only function is to produce 

 motion of the sperm and thus bring it in contact with the egg. 

 The sperms of some animals, however, have no motile tail and 

 are brought into contact with the egg by other means. 



The important conclusion to be drawn from this description 

 of the origin and structure of eggs and sperms is, that they 

 are essentially equivalent to each other. Even though the egg 

 is very large and the sperm is very small, and though the egg 

 is motionless and the sperm is commonly endowed with motion, 

 so far as concerns their most essential parts they are identical. 

 Each contains one nucleus, with chromosomes equivalent to 

 half the amount present in the ordinary cells of the organisms 

 from which these cells were derived; each may contain a centro- 

 some, though this is not always found. The eggs contain food 

 upon which the young embryo feeds, and the sperm possesses 

 a tail by which it can swim; but these are secondary features, 

 and in essential characters the egg and sperm are identical. 



Entrance of the Sperm into the Egg. When the sperms are 

 mature they are excreted through the ducts of the spermaries 

 to the exterior. If not excreted into the water, as is frequently 

 the case with water animals, a quantity of liquid is sometimes 

 excreted with them, in which the cells can swim by their motile 

 tail. All organisms have some method by which the sperms 

 and eggs are brought together. Sometimes both of them are 

 thrown in large numbers into the water and depend upon 

 chance currents to bring them together. Among many of the 

 higher animals there are developed special copulatory organs, 

 whose function is to bring the eggs and sperms together. Among 



