REPRODUCTION 509 



and erect. The coordinating center of erection lies in the lumbar 

 region of the spinal cord, and may be excite^ reflexly by mechan- 

 ical stimulation of the penis, or under tfcj influence of nervous 

 impulses originating in the brain and associated with sexual emo- 

 tions. The corpus spongiosum resembles the corpora cavernosa 

 in essential structure and function. 



The skin of the penis is thin and forms a simple layer for some 

 distance; towards the end of the organ it separates and forms a 

 fold, the foreskin or prepuce, which doubles back, and, becoming 

 soft, moist, red, and very vascular, covers the glans to the meatus 

 urinarius, where it becomes continuous with the mucous mem- 

 brane of the urethra; in it, near the projecting posterior rim of 

 the glans, are embedded many sebaceous glands. It possesses 

 nerve end organs (genital corpuscles) which much resemble end 

 bulbs in structure. 



The Seminal Fluid. The essential elements of the testicular 

 secretion are much modified cells, the spermatozoa, which are 

 passed out with some albuminous liquid. The spermatozoa 

 (Fig. 150) are motile bodies about 0.04 mm. (^ J 7 inch) in length. 

 They have a flattened clear body or head and a 

 long vibratile tail or cilium; the portion of the 

 tail nearest to the head is thicker than the rest, 

 and is known as the neck. The mode of develop- 

 ment of a spermatozoon shows that the head is a 

 cell-nucleus and the neck and tail a modified cell- 

 body. FIG. 150. Sper- 



On cross-section a seminiferous tubule pre- 



sents externally a well-marked basement mem- side view, a, head; 



, , . T , i i f > neck; c, tail. 



brane, upon which are borne several layers of 

 cells; the lumen or bore of the tubule is in great part occu- 

 pied by the tails of spermatozoa projecting from some of the 

 lining cells. The outer cells, those next the basement membrane, 

 are arranged in a single layer, and are usually found in one or 

 other stage of active karyokinetic division (p. 19). The result of 

 the division is an outer cell, which remains next the basement 

 membrane to repeat the process, and an inner, which is the mother- 

 cell of spermatozoa. The latter cell by repeated mitotic division 

 gives rise to four cells lying side by side and each having a rela- 

 tively large nucleus and small cell-body. These cells elongate, 



