CHAPTER X. 



THE CONSTRUCTION OF THE BODY FROM THE GERM- 

 LAYERS. 



Hie Original (Palingenetic) Development of the Vertebrate Body from 

 the Gaetrula. — 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 Ccelom 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 Walle. 



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

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



