RELATION OF THE OBGANS TO THE GERM-LAYERS. 27 1 



whole trunk, or the flesh, surrounding the vertebral column, 

 and consisting of two main groups of muscles ; the dorsal, 

 or upper side-muscles of the trunk, and the ventral, or lower 

 side-muscles of the trunk. To these must be added, in the 

 third place, the inner skeleton, which is especially character- 

 istic of Vertebrates, the central formation of which is the 

 spinal axis or notochord, developing at a later period 

 into the articulated vertebral column; also all the bones, 

 cartilages, ligaments, etc., which form the vertebral skeleton 

 in all more highly developed Vertebrates, and are connected 

 by the sinews and muscles belonging to it. Fourthly and 

 finally, from the innermost layer of cells of this secondary 

 germ-layer develops the exocoelar, that is, the outer, or 

 parietal ccelom-epithelium, the cell-layer which forms the 

 inner covering of the body- wall, and which is also probably 

 the original site of the male sexual cells. 



The third secondary germ-layer is the intestinal-fibrous 

 layer. From this is developed, firstly, the endoccelar, that 

 is, the inner, or visceral ccelom-epithelium, the layer of 

 cells, covering the outer surface of the whole intestine, pro- 

 bably also the site of the female sexual cells. Secondly, from 

 this layer originates the heart, and the great blood-vessels 

 of the body, as well as the blood itself, so that it has been 

 called, in a peculiar sense, the vascular layer. The great 

 blood-channels, or arteries, going from the heart and the 

 great veins passing to the heart, as well as the chyle-ves- 

 sels, which open into the latter, are formed, like the heart, 

 the lymph, and the blood itself, from this third germ- 

 layer. Thirdly, arises the muscular tube of the intes- 

 tines, or the mesenteric tube, that is, the whole of those 

 fibrous and fleshy parts which form the outer wall of the 



