70 



GENERAL TRIXCirLES OF ZOOLOGY 



(fig. 33), of the gut, of tlie boil_\' ca\ily, or of parts cut olT from this 

 (fig. 34). This connection of the sexual cells with the epithelium 

 has a deeper meaning since man)- organisms, particularly those of low 

 structure, consist e.xclusively of epithelia and therefore must develop 



Fig. ^v — Germinal epithelium of a medusa, ck. ectoderm; 01, entoderm; 0, egg; 



e, cpiUielium. 



their sexual products in epithelium. In other words, sexual and epithelial 

 cells are the oldest elements of the animal Ixxly, and hence very early 

 came into relation with one another. 



Sexual epithelium (or gcniiiiuil cpillu-Hiiiii) like glandular epithelium 

 has a tendencv to grow into the subepithelial tissues in the form of 



Fig. 34. — Section llirougli llio ovary of a nc\v4ioni cliil I (after W'alilcycr'). i;c, 

 germinal cpitheliuiu; /jc, priniiuw ei;.^ in die jj;riniinal i'])ilhidium; />, egg-])Oueh; i,^, 

 egg-nest constricted oft from the pouehlike gi\n\ ih {p); /', single egg with follicle; 

 1', blood-vessel. 



isolated or branching tubes (figs. 34, p, 35), and thus in many groups 

 of animals the sexual organs resemble branched glands; hence one speaks 

 as often of sexual glands as of sexual organs (fig. 34). The male and 

 female cells, the specific elements of the germinal epithelia and of the 



