CHAPTER X. 



THE CONSTRUCTION OF THE BODY FROM THE GERM- 

 LAYERS. 



The Original (Palingenetic) Development of the Vertebrate Body from 

 the Gaatrula. Relation of this Process to the Later (Kenogenetic) 

 Germination, as it occurs in Mammals. The most important act in the 

 Formation of the Vertebrate. The Primary Germ-layers, and also the 

 Secondary Germ-layers, which arise by Fission of the Primaries, 

 originally form Closed Tubes. Contemporaneously with the Completion 

 of the Yelk-sac, the Germ-layers flatten, and only later again assume 

 a Tubular Form. Origin of the Disc-shaped Mammalian Germ-area. 

 Light Germ-area (area pellucida) and Dark Germ-area (area 

 opaca). The Oval Germ-shield, which afterwards assumes the Shape 

 of the Sole of a Shoe, appears in the Centre of the Light Germ-area 

 (a. pellucida). The Primitive Streak separates the Germ-shield into 

 a Eight and Left Half. Below the Dorsal Furrow the Central Germ- 

 layer parts into the Notochord and the Two Side-layers. The Side- 

 layers split horizontally into Two Layers : the Skin-fibrous layer and 

 the Intestinal-fibrous layer. The Primary Vertebral Cords separate from 

 the Side-layers. The Skin-sensory Layer separates into Three Parts : 

 the Horny Layer, Spinal Canal, and Primitive Kidney. Formation of 

 the Coelom and the First Arteries. The Intestinal Canal proceeds from 

 the Intestinal Furrow. The Embryo separates from the Germ- vesicle. 

 Around it is formed the Amnion-fold, which coalesces over the back 

 of the Embryo, so as to form a Closed Sac. The Amnion. The 

 Amnion-water. The Yelk-sac, or Navel-vesicle. The Closing of the 

 Intestinal and Ventral Walls occasions the Formation of the Navel. 

 The Dorsal and Ventral Walls. 

 " The development of the Vertebrate proceeds from an axis upward, in 



two layers, which coalesce at the edges, and also downward, in two layers, 



