70 



GENERAL PRINCIPLES OF ZOOLOGY 



(fig. 33), of the gut, of the body cavity, or of parts cut off from this 

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

 has a deeper meaning since many organisms, particularly those of low 

 structure, consist exclusively of epithelia and therefore must develop 



FIG. 33. Germinal epithelium of a medusa, ek, ectoderm; en, entoderm; o, egg; 



e, epithelium. 



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

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

 came into rela.tion with one another. 



Sexual epithelium (or germinal epithelium} like glandular epithelium 

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



FIG. 34. Section through the ovary of a new-born child (after Waldeyer). ge, 

 germinal epithelium; pe, primitive egg in the germinal epithelium; p, egg-pouch; g, 

 egg-nest constricted off from the pouchlike growth (p) ; /, single egg with follicle ; 

 v, blood-vessel. 



isolated or branching tubes (figs. 34, />, 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 



