TESTES. 229 



(Hst.), which is 45-60^ long; and a thinner end segment 

 (Est.\ 6-10 IJL in length. The axis runs through the whole 

 length of the tall, and a finely fibrillar structure can be recog- 

 nized throughout. The capsule is lacking in the end segment, 

 but is considerably thickened in the rest of the tail. Some 

 authors have described an undulating membrane running the 

 whole length of the tail. In many animals the spermatozoon 

 shows a much more complicated structure than that described. 



By means of a lashing, waving motion of the tail, the sper- 

 matozoa can change their location with considerable rapidity. 

 They are quite resistant to low temperature, although their 

 motility becomes very slight in temperatures much lower than 

 that of the body. They can, however, regain their activity 

 even after a considerable period of cooling. Alkaline fluids 

 tend to increase the motility, while acids inhibit the activity 

 and finally kill the spermatozoa. 



The development of the spermatozoa is fairly well known in 

 some animals. The process in mammals has been studied especi- 

 ally by Ebner, v. Leuhossek, Hermann, Meves, v. La Valette, 

 and others. In the walls of the seminiferous tubules there are 

 two kinds of cell elements: the essential gland cells, which play 

 a direct role in the formation of the spermatozoa, and the so- 

 called supporting cells (cells of Sertoli). The latter are large 

 membraneless cells, which lie always with their bases on the 

 membrana propria and processes extending inward between the 

 essential cells. They possess a large, clear, flattened nucleus, 

 which is somewhat triangular on section. They are supposed 

 by many authors to assist in the nourishment of the developing 

 spermatozoa (v. Ebner, Benda, Plato, K. Peter, and others). 



The gland cells, which finally give rise to the spermatozoa, 

 are arranged in many layers, those nearest the lumen being the 

 youngest and most nearly related to the spermatozoa themselves 

 (Fig. 176). The whole process of spermatogenesis begins in 

 the most peripherally lying cells, the so-called spermatogonia. 

 These are low columnar cells lying on the basement membrane. 

 They increase in number by mitotic division, and give rise to 

 new spermatogonia, which become situated in a row by them- 



