248 AN INTRODUCTION TO ZOOLOGY 



While the caudal part of the mesonephros in the male is similar 

 to that in the female, the anterior end is very different. It is larger 

 than in the female, but almost completely hidden from view by the 

 highly convoluted mesonephric duct that is closely applied to its 

 ventral surface. In the embryo it is concerned with excretion, but 

 in the adult this activity is lost and it is functional in secreting a 

 nutritive fluid for the sperms, consequently it is sometimes termed the 

 epididymis. 



The mesonephric duct in the female is a straight fairly simple 

 tube, commencing at the front end as a thin vessel which runs back- 

 wards on the ventral face of the kidney. In the hinder part it en- 

 larges to form a urinary sinus into which open the secondary ureters, 

 about six in number, and this unites with its fellow to constitute 

 a small median urinary sinus opening upon the urinary papilla. The 

 duct is concerned with the conveyance of urine to the exterior, so 

 that it constitutes a ureter. 



In the male the anterior portion of the mesonephric duct is much 

 enlarged and bent upon itself. Into it opens the tubules, enlarged, 

 devoid of Malpighian bodies and with their epithelium considerably 

 modified. Near the line of demarcation between the cranial and 

 caudal mesonephros the bends become fewer, and it passes on swelling 

 out to form a seminal vesicle or vesicula seminalis. After receiving 

 the urinary vessels at its lower end it unites with that of the opposite 

 side to form a small median urogenital sinus whose opening is situated 

 upon the urogenital papilla. The sinus on each side is produced 

 into a forwardly directed long sac-like diverticulum, the sperm sac, 

 which lies closely attached to the ventral surface of the seminal 

 vesicle. The first four or five of the secondary ureters do not open 

 into the mesonephric duct directly, as in the female, but into an acces- 

 sory tube often termed " the ureter " which joins the seminal vesicle 

 near its hinder end. Behind this, again, five or six secondary ureters 

 draining the hinder extremity of the kidney open separately. These 

 secondary ureters, the sperm sacs and the seminal vesicle, cover a 

 large part of the ventral surface of the kidney. Thus it will be 

 seen that in the male the mesonephric duct serves for the greater 

 part of its length for the conveyance of the spermatic fluid, and so 

 constitutes a vas deferens, and only the last part of it is concerned 

 with the transference of urine. 



In the female embryo an ovary starts to develop on each 

 side, but that on the left soon disappears, so that in the adult we 

 find only one ovary present, that of the right side. It is a con- 

 spicuous structure lying slightly to the right of the middle line along 

 the median half of the coelom, to the dorsal wall of which it is attached 

 by a fold of peritoneum, the mesovarium. Its surface is marked 



