ANUBIDA. 403 



is formed by the union of the two vasa deferentia ; it is 

 roughly diamond-shaped when viewed in transverse 

 section, and is lined internally by a chitinous cuticle 

 which rests on an epithelial layer containing numerous 

 nuclei. The ductus is covered externally by a coat of 

 connective tissue. Dilator muscles arise from its chitinous 

 lining and, passing in an oblique direction outwards, are 

 inserted into the body wall (muse). 



The lumen of the testes is filled with a dense mass of 

 nucleated tissue, which consists of different stages in the 

 development of the spermatozoa. At the apex of the 

 gland the testis-wall becomes drawn out into the terminal 

 filament (fig. 58). The filament is tubular in structure, 

 and numerous nuclei can be seen in its walls. Xear to its 

 apex the cellular wall dwindles away, and the filament 

 becomes reduced to merely a thread-like prolongation of 

 the connective tissue coat of the testis. It is purely liga- 

 mentous in function, and serves to maintain the testis in 

 position in the body-cavity. 



If the structure of the testes be examined from the 

 apex of the organ and be followed downwards near to 

 where the efferent duct is given off, the successive stages 

 in the development of the spermatozoa can be readily 

 traced. The germinal tissue is in the form of a mass or 

 ridge, situated at about the middle of the length of the 

 testis. It consists of a mass of cells (fig. 59a), which are 

 seen in places to be roughly polygonal in shape ; each cell 

 contains a large, very deeply staining nucleus. The 

 germinal nuclei divide, and the direct products of the 

 division fill almost the whole of the anterior third of each 

 testis (d. s. in figs. 58 and 59a). The nuclei are seen to 

 principally arrange themselves in groups, which are more 

 or less ovoid or circular in form, and contain an area of 

 clear protoplasm in their centres. Traced further back- 



