28O THE EVOLUTION OF MAN. 



their immediate neighbourhood are the sexual organs, in the 

 form of simple heaps of cells, which are attached to the 

 body- wall, near the mesentery. Presumably, they originated 

 as hermaphrodite glands, the female gland (y) from the 

 inner, the male gland (x) from the outer germ-layer. The 

 former becomes the ovary, the latter the testes. Simul- 

 taneously with these changes, the spinal tube has completely 

 detached itself from its original site, the skin-sensory layer, 

 and has made its way far into the body. A sheath has 

 formed round the notochord, and processes from this "noto- 

 chord sheath " grow round the spinal tube, imbedding it in 

 a vertebral canal (Fig. 68, 69 w). 



If, for a moment, we leave the transverse sections, and 

 trace the evolution of the primitive Vertebrate in longi- 

 tudinal sections, we see that at a very early period the 

 intestinal tube is divided into a gill-intestine and a stomach- 

 intestine, in consequence of the appearance of gills in the 

 anterior portion. The primitive mouth of the Gastrula 

 closes ; the two permanent openings of the future intestine 

 arise as new formations from the exterior ; the mouth in 

 front, the anus behind. Moreover, the outer body-wall 

 becomes articulated, owing to the fact that the fleshy mass 

 of the trunk-muscles assumes the form of a number of 

 similar, consecutive portions, segments, or metamera. In 

 correspondence with these, each of the respective portions 

 of the nerve and vascular systems becomes distinct. 



The following processes must, therefore, be emphasized 

 as the chief acts by which the simple Gastrula changes into 

 the typical vertebrate organism : 1. The two primary germ- 

 layers part by fission into four secondary germ-layers. 

 2. The Gastrula becomes flattened, so that, instead of a form 



