STUDIES ON THE ECTOPARASXTIC TEEMATODES OF JAPAN. 85 



Each testis is separated from its neighbours by a more or less thin 

 layer of mesenchyma, which in this region assumes the character of 

 reticulated, fibrous connective tissue (PI. V, fig. 7 ; PL XI, fig. 3 ; 

 PI. XXIII, fig. 7 ; PI. XXIV, fig. 2) ; and is usually destitute of 

 any distinct epithelium. The testes are also traversed by the dorso- 

 ventral muscular fibres ; but the greater part of these pass between 

 them through the mesenchymatous septa just mentioned. The con- 

 tents of the testes consist of sperm cells in various stages of develop- 

 ment, scattered without any regularity. In some species, as in 

 Diclidopliora sessilis (PI. XI, fig. 5), I have often observed cells with 

 large nucleus arranged in a single layer on the wall of the testis ; 

 but these seemed not to form a permanent epithelium. For, they 

 were only loosely apposed to the mesenchyma, and in many testes, 

 especially in those in which the greater part of the sperm cells had 

 finished their development, they were wholly absent. A general view 

 of the contents of the testes may be obtained from fig. 7, PI. V, figs. 3 

 & 5,. PL XI, fig. 7, PL XXIII, and fig. 2, PL XXIV. The most 

 conspicuous elements besides the already developed spermatozoa are the 

 groups of large nuclei containing a certain number (how many I have 

 not been able to make out with satisfaction) of chromatin granules, the 

 interspaces of which stain uniformly but far more weakly with 

 haematoxylin, and the cells of large dimensions usually more or less 

 of a spherical form, with a single, long thread of chromatin forming an 

 irregular skein, or with numerous, more or less lozenge-shaped pieces 

 of chromatin. The large nuclei just mentioned were usually sur- 

 rounded by such a scanty layer of protoplasm as almost to look naked. 

 Besides these there are also groups of much smaller nuclei imbedded 

 together in a uniform mass of very finely granular protoplasm, which 

 sometimes showed traces of separation corresponding to each nucleus. 



The various elements above characterised are obviously stages in 



