39^ THE EVOLUTION OF MAN. 



originate from the intestinal layer, the sperm-cells from the 

 skin-lay er.^^^ In Gastrgeads, Sponges, and Hydro-medussB 

 this appears really to be the case. The development of the 

 sexual differences, which is so rich in results, must, ac- 

 cordingly, have commenced even during the differentiation 

 of the two primary germ-layers in the simplest and lowest 

 Plant Animals ; the exoderm would be the male germ-layer, 

 the entoderm, the female. If this discovery of Van Beneden 

 is established and proves to be a universal law, Biology will 

 gain a most pregnant advance ; for not only would all the 

 contradictory empiric explanations be answered, but a new 

 path would be opened for philosophic reflection on one of 

 the most important of biogenetic processes. 



If we now trace the Phylogeny of the sexual organs 

 in our earliest Metazoic ancestors further, as it is indicated, 

 at the present time, in the Comparative Anatomy and 

 Ontogeny of the lowest Worms and Plant Animals, we 

 note, as the first advance, the accumulation of the cells of 

 both sexes into definite groups. While in Sponges and 

 the lowest Hydra-Polyps single scattered cells separate from 

 the cell-layers of the two primary germ-layers, and become 

 isolated and free sexual cells, in the higher Plant Animals 

 and Worms we find these same cells associated and col- 

 lected into groups of aggregate cells, which are, hence- 

 forward, called " sexual glands," or " germ-glands " {gonades). 

 It is only now that we can speak of sexual organs in the 

 morphological sense. The female germ-glands which, as 

 such, in their simplest form constitute a mass of homo- 

 genous egg-cells, are the ovaries {ovaria, or oophora; Fig. 

 211, e, p. 198). The male germ -glands, which in their 

 primitive form also consist merely of a mass of sperm-cells. 



