CHAPTER VIII. 



EGG CLEAVAGE AND THE FORMATION OF THE GERM- 

 LAYERS. 



First Processes after the Fertilization of the Egg-cell is complete. Original 

 or Palingenetic Form of Egg-cleavage. Significance of the Cleavage- 

 process. Mulberry-germ, or Morula. Germ-vesicle, or Blastnla. Germ- 

 membrane, or Blastoderm. Inversion (Invagination) of the Germ-vesicle. 

 Formation of the Gastrnla. Primitive Intestine and Primitive 

 Month. The Two Primary Germ-layers; Exoderm and Entoderm. 

 Kenogenetic Form of Egg-cleavage. Unequal Cleavage (aegmentatio 

 inequalis) and Hood-gastrula (Amphigastrula) of Amphibia and 

 Mammalia. Total and Partial Cleavage. Holoblastic and Meroblastic 

 Eggs. Discoidal Cleavage (segmentatio discoidalis) and Disc-gastrula 

 (Disrogastrula) of Fishes, Reptiles, Birds. Superficial Cleavage (seg- 

 mentatio superficialis) and Vesicular Gastrula (Peri-Gastrula) of Ar- 

 ticulates (Arthropoda). Permanent Two-layered Body-form of Lower 

 Animals. The Two-layered Primaeval Parent.form ; Gastrgea. 

 Homology of the Two Primary Germ-layers in all Intestinal Animals 

 (Metaz''a). Significance of the Two Primary Germ-layers. Origin 

 and Significance of the Four Secondary Germ-layers. The Exoderm 

 or Skin-layer gives rise to the Skin-sensory Layer and the Skin- 

 fibrous Layer. The Entoderm or Intestinal Layer gives rise to the 

 Intestinal-fibrous Layer and the Intestinal-glandular Layer. 



" The distinguishing of the strata, or layers, in the embryonic membrane 

 was a turning-point in the study of the history of evolution, and placed 

 later researches in their proper light. A division of the (disc-shaped) 

 embryo into an animal and a plastic part first takes place. When this 

 division is complete, each part has two layers. In. the lower part (the 

 plastic or vegetative layer) are a serous and a vascular layer, each of pecu- 



