180 HUMAN PHYSIOLOGY. 



tatic tubules. The gland secretes a fluid which forms part of the semen, 

 and assists in maintaining the vitality of the spermatozoa. 



Semen is a complex fluid, made up of the secretions from the testicles, 

 the vesiculae seminales, the prostatic and urethral glands. It is grayish- 

 white in color, mucilaginous in consistence, of a characteristic odor, and 

 somewhat heavier than water. From half a drachm to a drachm is ejacu- 

 lated at each orgasm. 



The Spermatozoa are peculiar anatomical elements, developed within 

 the seminal tubules, and possess the power of spontaneous movement. 

 The spermatozoa consist of a conoidal head and a long filamentous tail, 

 which is in continuous and active motion; as long as they remain in the 

 vas deferens they are quiescent, but when free to move in the fluid of the 

 vesicuke seminales, become very active. 



Origin. The spermatozoa appear at the age of puberty, and are then 

 constantly formed until an advanced age. They are developed from the 

 nuclei of large, round cells contained in the anterior of the seminal tubules, 

 as many as fifteen to twenty developing in a single cell. 



When the spermatozoa are introduced into the vagina, they pass readily 

 into the uterus and through the Fallopian tubes toward the ovaries, where 

 they remain and retain their vitality for a period of from 8 to 10 days. 



Fecundation is the union of the spermatozoa with the ovum during its 

 passage toward the uterus, and usually takes place in the Fallopian tube, 

 just outside of the womb. After floating around the ovum in an active man- 

 ner, they penetrate the vitelline membrane, pass into the interior of the 

 vitellus, where they lose their vitality, and along with the germinal vesicle 

 entirely disappear. 



DEVELOPMENT OF ACCESSORY STRUCTURES. 



Segmentation of the Vitellus. After the disappearance of the 

 spermatozoa and the germinal vesicle there remains a transparent, granular, 

 albuminous substance, in the centre of which a new nucleus soon appears; 

 this constitutes the parent cells, and is the first stage in the development of 

 the new being. 



Following this, the vitellus undergoes segmentation; a constriction 

 appears on the opposite side of the vitellus, which gradually deepens, 

 until the yelk is divided into two segments, each of which has a distinct 

 nucleus and nucleolus; these two segments undergo a further- division into 

 four, the four into eight, the eight into others, and so on, until the entire 



