12 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



perforatorium by means of which the spermatozoon penetrates the ovum. 

 The head contains the nuclear elements of the sperm cell. The disc- 

 shaped neck includes the anterior centrosomal body. The tail begins with 

 the posterior centrosomal body and is divided into a short connecting piece , 

 a chief piece, or flagellum, which forms about four-fifths of the length of the 

 sperm cell, and a short end piece, or terminal filament. The connecting 

 piece is marked off from the chief piece by the annulus. The connecting 

 piece is traversed by the axial filament (filum principale) , and is surrounded : 

 (i) by the sheath common to it and to the flagellum; (2) by a sheath con- 

 taining a spiral filament; and (3) by a mitochondrial sheath. The chief 

 piece is composed of the axial filament surrounded by a cytoplasmic sheath, 

 while the end piece comprises the naked continuation of the axial filament. 

 The spermatozoa are motile, being propelled by the movements of the 

 tail. They swim always against a current at the rate of about 2.5 mm. a 

 minute. This is important, as the outwardly directed currents induced by 

 the ciliary action of the uterine tubes and uterus direct the spermatozoa 

 by the shortest route to the infundibulum. Keibel has found spermatozoa 

 alive three days after the execution of the criminal from whom they were 

 obtained. They have been found motile in the uterine tube three and 

 one-half weeks after coitus and have been kept alive eight days outside 

 the body by artificial means. It is not known for how long a period 

 spermatozoa are capable of fertilizing ova. Keibel holds that this would 

 certainly be more than a week. However, Lillie (1915) has shown with 

 sea urchins that the ability to fertilize is lost long before vitality or motil- 

 ity is impaired, and Mall (1918) concludes that the duration of the fertil- 

 izing power of human spermatozoa is safely less than the corresponding 

 period in the ovum which is "probably for fully 24 hours after ovulation." 

 Lode estimates that 200 million spermatozoa are liberated at an average 

 ejaculation. 



MITOSIS AND AMITOSIS 



All cells arise from pre-existing cells by division. There are two 

 methods of cell division amitosis and mitosis. 



Amitosis. -Cells may divide directly by the simple fission of their 

 nuclei and cytoplasm. This rather infrequent process is called amitosis. 

 Amitosis is said by many to occur only in moribund cells. It is the type 

 of cell division demonstrable in the epithelium of the bladder. 



Mitosis. In the reproduction of typically active somatic cells and in 

 all germ cells, complicated changes take place in the nucleus. These 

 changes give rise to thread-like structures, hence the process is termed 

 mitosis (thread) in distinction to amitosis (no thread) . Mitosis is divided 

 for convenience into four phases (Fig. 8). 



