91 



Sections through the testis show a tubular 

 structure, the tubes opening into one another until 

 finally a small duct is formed which joins others and 

 makes up the canal opening into the vas deferens. 

 In the ripe testis the germinal epithelium, which forms 

 the wall of these tubes, gives rise to sj^miatocytes, which 

 break away and fill up the marginal area of the cavity 

 (fig. 56). Nearer the centre of the cavity are numerous 

 smaller cells, the products of division of the spermato- 

 cytes, and themselves the spermatids (fig. 56, Sptd.). 

 Both spermatocytes and spermatids are characterised by 

 the small quantity of cytoplasm and the densely-stained 

 nuclei. Complete series of stages in spermatogenesis 

 may thus be seen, but the bodies formed are not of such 

 a size as to make observation easy. The nucleus of 

 the spermatid gradually elongates and becomes more 

 attenuated to form the head of the spermatozoon. 



Retzius was the first to discover two kinds of 

 spermatozoa in the whelk. The presence of two kinds of 

 spermatozoa in the gastropoda was noticed so far back as 

 1875 by Schenk in Murex. Since then the double 

 character has been recognised in several species of 

 Prosobranchs. The two different forms have been 

 designated by the names " hair-like " and " worm-like." 

 The former are long fibres with an extremely long head, 

 almost half the length of the sperm (fig. 57, b.). In 

 ordinary stained preparations these will be probably the 

 only sperms recognised, and the only differentiation will 

 be into the long narrow head and tail. At the anterior 

 end of the head is a small, clear, vesicular cap. From 

 this point a central fibre is visible running down to the 

 base of the head. Very often in fixation this contracts 

 so as to throw the head into a spiral. A middle piece 

 follows the head, but is of the same diameter. The tail 



G 



