ORIGIN OF THE SEXUAL CELLS. 397 



progenitors of the sperm-cells and of the egg-cells, with- 

 drew, during the separation of the skin-fibrous layer from 

 the skin-sensory layer, or of the intestinal-fibrous layer 

 from the intestinal-glandular layer, into the body-cavity 

 cceloma), which was in process of formation; and that 

 they thus acquired the internal position between the two 

 fibrous layers, which appears as their original position, 

 when the sexual cells first become distinct in the vertebrate 

 embryo. Otherwise, we should be obliged to accept the 

 improbable polyphyletic hypothesis, that the origin of the 

 egg-cells and sperm-cells is different in the higher and in 

 the lower animals, that their origin in the former is inde- 

 pendent of that in the latter. 



If we, accordingly, derive the two kinds of sexual cells 

 from the two primary germ-layers in man as in all other 

 animals, the farther question arises : Did the female egg- 

 cells and the male sperm-cells develop from both primary 

 germ-layers, or from one only ? and, in the latter case, from 

 which of the two ? This important and interesting question 

 is one of the most difficult and obscure problems in the 

 history of evolution, and, up to the present moment, no full 

 and clear solution has been attained. On the contrary, 

 the most opposite answers are given to it even yet by 

 naturalists of note. Among the various possible solutions 

 only two have been generally considered. It has been 

 supposed that both kinds of sexual cells originally de- 

 veloped from the same- primary germ-layer, either from the 

 skin-layer or the intestinal layer ; but almost as many and 

 as able observers have accepted the one as the origin as 

 the other. Quite recently the Belgian naturalist, EduarJ 

 van Beneden, has asserted, on the contrary, that the egg-cells 



