THE HORMONES IN HUMAN REPRODUCTION 



ing the testis with acid and then, with incredible patience and 

 dexterity, dissected out complete tubules under the micro- 

 scope with fine needles and mounted them, without breakage, 

 on glass slides. He found that they are arranged in loops or 

 arches, all opening into a network of channels at the hilum a 

 of the testis, whence they are drained off by a dozen larger | 

 channels into the epididymis. All these facts have been de- 

 picted in Fig. 31. 



In cross section under high magnification (Plate XXIV, 

 A ) the tubules are seen to be lined by several layers of cells. 

 The outermost layer (i.e. that farthest from the central 

 channel of the tubule) is made up of large clear cells. As these 

 divide to form the next and succeeding layers, the cells be- 

 come smaller. Finally little is left except the nucleus, and 

 even this becomes more compact. A long tail-like process 

 grows out of the rapidly shrinking cell. This figure represents 

 the rat's sperm cells with their hook or halberd-shaped heads. 

 The completed human sperm cell is shown in Fig. 7. It has 

 a total length, including the tail, of about 60 microns or 

 1/400 inch. The head is about 5 microns long by 3 wide. The 

 sperm cell is therefore by far the smallest cell in the body. 

 An idea of its relative dimensions may be gained by comparing 

 it with the printed period at the end of this sentence. If the 

 sperm heads were laid like a pavement, one layer deep, on such 

 a dot, it would take about 2,500 of them to cover it. About 

 12 eg^ cells could be placed on such an area. 



As the sperm cells are formed, little clumps of them cling 

 to supporting cells in the lining of the tubules until finally 

 they drop off and are carried along the channel toward the 

 seminal ducts. In animals that breed at all seasons of the 

 year, for instance man, rat, rabbit and guinea pig, sperm 

 production goes on continuously, passing in waves along the 

 tubules, so that sperm cells are always available. Many 

 wild species, however, have distinct breeding seasons once a 

 year, and in these there is a cycle of testicular activity. In 



f 220 } 



