22 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



It is traversed by the axial filament (filum principale), and surrounded (i) by 

 the sheath common to the flagellum; (2) by a sheath containing a spiral filament; 

 and (3) by a mitochondria sheath. The flagellum is composed of an axial filament 

 surrounded by a cytoplasmic sheath and the end-piece is 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 25 micra per second, or 

 i mm. every forty seconds. This is important, as the outwardly directed cur- 

 rents induced by the ciliary action of the uterine tubes and uterus direct the sper- 

 matozoa by the shortest route to the infundibulum. Keibel has found sperma- 

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

 obtained. They have been found motile in the vagina twelve to seventeen days 

 after coitus. They have been kept alive eight days outside the body by arti- 

 ficial means. It is not known for how long a period they may be capable of fer- 

 tilizing ova but, according to Keibel, this period would be certainly more than a 

 week. 



MITOSIS AND AMTTOSIS 



Before the discharged ovum can be fertilized by the male germ cell, it must 

 undergo a process of cell division and reduction of chromosomes known as matu- 

 ration. As the student may not be familiar with the processes of cell division, 

 a brief description may be necessary. (For details of mitosis see text-books of 

 histology and E. B. Wilson's "The Cell".) 



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

 cytoplasm. This process is called amitosis. Amitosis is said to occur only in 

 moribund cells. It is the type of cell division found in the epithelium of the 

 bladder. 



Mitosis. In the reproduction of normally active 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). 



Prophase (Fig. 8, I-III). i. The centrosome divides and the two minute 

 bodies resulting from the division move apart, ultimately occupying positions at 

 opposite poles of the nucleus. 



2. Astral rays appear in the cytoplasm about each centreole. They radiate 

 from it and the threads of the central or achromatic spindle are formed between 

 the two asters, thus constituting the amphiaster (Fig. 8, II). 



