CHAPTER VIIL 



EGG- CLEAVAGE AND THE FORMATION OF THE GEEM- 



LAYEKS. 



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 Blastula. Germ- 

 membrane, or Blastoderm. — Invei-sion (Invagination) of the Germ-vesicle. 

 — Formation of the Gastrula. — Primitive Intestine and Primitive 

 Mouth. — The Two Primary Germ-layers ; Exoderm and Entoderm. — 

 Kenogenetic Form of Egg-cleavage. — Unequal Cleavage {aegmentatio 

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

 Mammalia. — Total and Partial Cleavage. — Holoblastic and Meroblastic 

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

 (Discogastrula) of Fishes, Eeptiles, Birds. — Superficial Cleavage (seg- 

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

 ticulates (Artliropoda) . — Permanent Tw^o-layered Body-form of Lower 

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

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

 (Metazoa). — 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- 



