330 



THE EVOLUTION OF MAN. 



and the four-layered stages). 3. The axial soldering, or 

 the coalescence of the germ-layers along the longitudinal 

 axis (giving rise to the axis-band). 4. The early sepa- 

 ration of the medullary tube from the skin-sensory layer 

 (by the formation of the dorsal furrow and the spinal 

 swellings). 5. The early origin of the primitive kidney 

 ducfcs (probably from the skin-sensory layer). 6. The early 

 division of the skin-fibrous layer into the chorda, the primi- 

 tive vertebral cords, and the trunk-muscle plates. 7. The 

 separation of the skin-fibrous layer from the intestinal- 

 fibrous layer (giving rise to the body-cavity, or coeloma). 

 8. The rudimentary primitive vessels, or aortse (from the 

 intestinal-fibrous layer). These important germ-processes 

 result in the formation of ten different parts of the body, 

 which we may call " the primitive organs," and which, in 

 the following list, are represented in their relation to the 

 germ-layers. (C£ Fig. 99, and Plate IV. Fig. 3.) 



Phylogenetic Jission of the germ-layers. 



Primitive Organt 

 {Fig. 99). 



Ontogenet >c fission 



of the 



germ-layers. 



Outer primary germ- 

 layer^ 



Skin-layer 



(Dermal laypr, or 

 £xoderma). 



B. 



Inner primary germ- 

 layer : 



Intestinal layer 



(Gastral lavf-r, or 

 Jintudervia). 



I. Secondary germ- 

 layer : 



Skin-sensory 

 layer. 



II. Secondary germ- 

 layer : 



Skin-fibrous 

 layer. 



'in. Secondary germ- 

 / layer : 



Intestinal- 

 fibrous layer. 



IV. Secondary germ- 

 layer : 



Intes'inal- 

 glandular layer. 



1. Ilorn-plate (/i). 



2. Medullary plate 



(mr). 



3. Primitive kidney 



(iung). 



4. Chorda (ch). 



5. Primitive verte- 



bral plate (wif). 



6. Skin-muscle plaie 



(hpl). 



1. Body-cavity (sp). 



8. Intestinal muscle 



plate (df). 



9. Primitive aorta 



(ao). 



10. Intestinal gland- 

 epithelium (cZ<i). 



A. Upper or 



Sensory layer, 



Kemak. 



B. Middle or 



Motor-germina' 



tive layer, 



Remak. 



C. Lower or 



Trophic layer, 



Kemak. 



