172 CELL DIVISION : DEVELOPMENT OF GERM-CELLS 



As this elongates it carries with it an extremely delicate layer 

 of cytoplasm. The thread becomes contractile, and in this 

 way the tail of the spermatozoon is formed, the centrosome 

 remaining at the root of the tail as the middle piece. The head 

 of the spermatozoon arises chiefly from the condensed nucleus 

 of the spermatid, and possesses a thin covering of cytoplasm. 



Examine prepared sections of the testis of the newt) frog, 

 or rat. 



C. Formation of the Zygote. 



If a mature ovum, as described above, be fertilised by a ripe 

 spermatozoon, the following changes take place. By the aid of 

 its vibratile tail the spermatozoon attaches itself to the ovum. 

 The head and middle piece pass into the cytoplasm of the egg, 

 leaving the tail behind. The ovum now contains two nuclei 

 and two centrosomes, belonging to the spermatozoon and ovum 

 respectively. The two centrosomes approach one another, and 

 between them is formed a spindle upon which the chromosomes 

 of the egg-nucleus and those of the sperm-nucleus are dis- 

 tributed. In some animals the paternal and maternal chromo- 

 somes are distinct, and in such cases it can be seen that each 

 set divides, and the four groups go to opposite poles of the 

 spindle two to each. The whole cell, or zygote, now divides, 

 so that each daughter-cell receives an equal number of both 

 paternal and maternal chromosomes. These being reduced by 

 the changes described on pp. 168-9, now by addition amount 

 to the full number characteristic of the species. 



