37 2 THE GENITOURINARY ORGANS. 



mu'cosa and the epithelium there is a basement membrane. There 

 occur in the urethra, beginning with the membranous portion, ir- 

 regularly scattered epithelial sacculations of different shapes. Some 

 of these show alveolar branching, and are then known as the glands 

 of Littre. 



The submucosa of the cavernous portion of the urethra, which 

 contains nonstriated muscle-tissue arranged circularly, is richly sup- 

 plied with veins, and contains pronounced plexuses communicating 

 with cavernous sinuses, which correspond in general to those of the 

 corpora cavernosa penis. 



The glans is covered by a layer of stratified squamous epithe- 

 lium, often possessing a thin stratum corneum (see Skin). Near 

 the corona of the glans penis there are now and then found small 

 sebaceous gland-s (see Hair), known as glands of Tyson. The pre- 

 puce is a duplication of the skin, the inner surface presenting the 

 appearance of a mucous membrane. 



The nerves terminating in the glans penis have recently been 

 studied by Dogiel, who made use of the methylene-blue method in 

 his investigation. He finds Meissner's corpuscles in the connective- 

 tissue papillae under the epithelium, Krause's spheric end-bulbs 

 somewhat deeper in the connective tissue, and the genital corpuscles 

 situated still deeper (see Sensory Nerve-endings). In the epithelium 

 are found free sensory nerve -endings. Pacinian corpuscles have 

 also been found in this region. 



4. SPERMATOGENESIS. 



In order that the student may obtain an understanding of the com- 

 plicated process of spermatogenesis we shall give a description of it 

 as it occurs in salamandra maculosa, which of all vertebrate animals 

 presents the phenomena in their simplest and best known form. 

 The student should understand, however, that many of the details 

 here described have not been observed in the testes of mammalia ; 

 and, since the spermatozoa of many of the mammalia are of simpler 

 structure than those of the salamander, the development of the 

 spermatozoa of the former is consequently simpler. It should also 

 be noticed that the general structure of the testes of the salamander 

 differs in some respects from that of the testes of mammalia, as given 

 in the preceding pages. 



At first the seminiferous tubules consist of solid cellular cords, 

 and it is only during active production of spermatozoa that a central 

 lumen is formed, in which the spermatosomes then lie. The cells 

 which compose these solid cords may be early differentiated into two 

 classes those of the one class being directly concerned in the pro- 

 duction of the spermatosomes ; those of the other appearing to have 

 a more passive role. The cells of the first class the spcrmatogo- 

 nia, or primitive seminal cells undergo a process of division accom- 

 panied by an increase in size. In this way they soon commence to 

 press upon the cells of the second class tMtfollicularoi sustentacu- 



