DEVELOPMENT OF THE SEXUAL ORGANS. 399 



are the testes (testiculi, or orchides; Fig. 211, A). We find 

 the ovaries and testes in this earliest and simplest shape 

 not only in many Worms (Annelida) and Plant Animals, 

 but also in the lowest Vertebrates, in the Skull-less Animals 

 (Acrania). In the anatomy of the Amphioxus we found the 

 ovaries of the female and the testes of the male consisting 

 of twenty to thirty elliptic or roundly four-cornered simple 

 sacs, of small size, attached to the inside of the gill-cavity 

 on each side of the intestine. (Of. vol. i. p. 425.) 



Only a single pair of germ-glands, lying far down in the 

 door of the body-cavity (Fig. 316, g), exist in all Skulled 

 Animals (Craniota), The first traces of these appear in the 

 ccelom-epithelium. Probably, in this case also, the male 

 sperm-cells originate from the skin-layer, the female egg- 

 cells, on the contrary, from the intestinal layer. The earliest 

 traces are visible in the embryo at the point where the 

 skin-fibrous layer and the intestinal-fibrous layer meet in 

 the middle plate (mesentery-plate) (Fig. 318, mp, p. 408). 

 At this very important point in the coelom-wall, where the 

 endocoelar (or visceral ccelom-epithelium) merges into the 

 exoccelar (or parietal ccelom-epithelium), in the embryo of 

 Man and the other Skulled Animals a small aggregation of 

 cells becomes visible, at a very early period, and this, accord- 

 ing to Waldeyer, 196 we may call the " germ- epithelium," or 

 (corresponding with the other plate-shaped rudiments of 

 organs) the sexual plate (Fig. 316, g ; Plate IV. Fig. 5,&). The 

 cells of this germ-plate, or sexual plate (lamella sexualis) are 

 essentially distinguished by their cylindrical form and by 

 their chemical constitution from the other cells of the 

 ccelom ; they are of quite different significance from the fiat 

 cells of the "serous coelom-epithelium " which line the 

 59 



