THE REPRODUCTIVE ORGANS. 343 



corresponding- testis : the duets then travel in the ventral surface 

 of the kidney towards its inner border, where they open into a 

 long-itudinal canal (Bidder), (Fig-. 222 I, L). Just before their 

 terminations each duet has an enlargement (I, C), the exact import 

 of which is unknown (see also Fig. 218 XV). 



The collecting-tubes open into the ureter at the hinder extremity 

 of the kidney. 



b. [Minute structure. The testis possesses a thin connective- 

 tissue capsule vuiderneath its peritoneal covering (Fig. 223 II, a); 

 this sends in trabeculae (Jj) towards the centre of the organ, and so 

 encloses the separate lobules. Each seminiferous tube arises from an 

 elongated irregular sinus placed towards the middle of the organ ; 

 the tubes (e) are slightly convoluted in their course towards the 

 periphery, near which they branch (Spengel). The tubes are from 

 0"i6 mm. to 0'i2 mm. in diameter (Kolliker), and are lined with 

 two kinds of cells (Fig. 222 III); the cells [x) nearer the periphery 

 are rounded and have large rounded nuclei, the diameters of which 

 vary from 0"0i3 to 0'02 mm.; these again possess large and 

 distinct nucleoli. The other cells {1/) are of elongated, spindle- 

 shaped form; and have oval nuclei, with an average length of 

 o*oi6 mm. and an average breadth of 0*005 mi^i. (Neu^mann). 



The rounded cells lie in groups which vary in thickness and 

 arrangement, and are often compressed so as to have polygonal 

 outlines. The spindle cells are arranged so as to radiate from the 

 lumen of the tube to the periphery, at an angle which varies from 

 45 to 90 degrees ; these are the spermatoblasts. 



The various changes which the spermatoblasts undergo in the 

 formation of spermatozoa will easily be understood by reference to 

 Fig. 223 I, a to k, where i and k rejjresent the fully developed 

 spermatozoa. These have three parts, head, middle part, and tail, 

 the respective lengths of which in the two species are, according to 

 Neumann, the following : — 



Head. , Middle part. Tail. 



Rana esculenta 0-0140 mm. 0*0025 iïi^ïî. 0*040 mm. 



Rana temporaria o-co66 „ 0*0330 ,, 0*045 m 



At the hilus of the testis the rounded cells of the seminiferous 

 tubes are gradually modified to form short cylindrical cells (o*oi 

 mm. long and o*oo6 mm. broad), the spindle cells being absent 

 (Neumann). The Vasa eferentia are also lined with a similar 

 columnar epithelium (Spengel).] 



