414 
Avgas persicus 
vas deferens consists of a tube, the diameter of which barely exceeds 
that of the Malpighian tubules; this narrow portion is disposed in a 
number of small convolutions near the extremities of the testis. The 
duct then undergoes a sudden and marked enlargement in calibre, and 
after running for a short distance in the direction of the lateral margin 
of the body, it loops round and continues its Course in an anterior 
direction, passing along the lateral margins of the accessory glands to 
a point situated some little distance posterior to the external genital 
opening, where it is again reflected backwards, and after running in 
the posterior direction for a short distance, turns in towards the middle 
line of the body and, with its partner of the other side, terminates the 
median seminal vesicle. 
The portion of the vas deferens lying between the narrow proximal 
section described above and the place of reflection near the external 
genital opening, is invariably of much greater diameter than the 
remainder of the duct, and as this dilated position is filled with 
spermatozoa, Heller termed this portion the Semenblase while he referred 
to the proximal narrow portion as the vas efferens 1 . The portion of the 
vas deferens lying between the dilated section and the median seminal 
vesicle, though considerably smaller in diameter than the middle section, 
is not nearly so narrow as the proximal section. This terminal section 
was described by Heller as the ductus deferens, who included the whole 
median unpaired terminal section—the seminal vesicle and the terminal 
portion of the genital canal—in the term ductus ejaculatorius. 
The wall of the vas deferens is thin, particularly in the dilated 
section. It is formed of an outer muscular sheath consisting of both 
circularly and longitudinally disposed fibres, and a lining epithelium of 
large flattened cells. The epithelial cells of the proximal narrow portion 
of the duct approach the columnar form, and are further distinguished 
by the fact that their boundaries are more clearly defined by cell 
membranes than is the case with the flattened cells which line the rest 
of the duct. 
The Seminal Vesicle. 
Plate XXVII, figs. 2-4, v. d. c. 
The seminal vesicle is formed by the union of the vasa deferentia; 
it is a thin-walled sac of triangular form situated in the middle of the 
body in a position corresponding to the uterus of the female, but is 
almost completely concealed from view by the lobes of the accessory 
1 Heller, C. (1858), pp. 317-320. 
