CONTENTS. IX 



TM99 



Standard of Measurement for the Grade of the State. — Parallel 

 between the Processes of Individual and of Race Development. — 

 The Functions of Evolution. — Growth. — Inorganic and Organic 

 Growth. — Simple and Complex Growth. — Nourishment and Change 

 of Substance. — Adaptation and Modification. — Reproduction. — 

 Asexual and Sexual Reproduction. — Heredity. — Division of Labour, 

 or Differentiation. — Atavism, or Reversion. — Coalescence. — The 

 Functions of Evolution as yet very little studied by Phyniology, 

 and hence the Evolutionary Process has often been misjudged. — 

 The Evolution of Consciousness, and the Limits to the Knowledge 

 of Nature. — Fitful and Gradual Evolution. — Fertilization. — Sexual 

 Generation. — The Egg-cell and the Sperm-cell. — Theory of the 

 Sperm-animals. — Sperm-cells a form of Whip-cell. — Union of the 

 Male Sperm-cell with the Female Fgg-cell. — The Product of this is 

 the Parent-cell, or Cytula. — Nature of the Process of Fertilization. 

 — Relation of the Kernel (Nucleus) to this Process. — Disappear- 

 ance of the Germ-vesicle. — Monerula. — Reversion to the Monera- 

 form. — The Cytula ... ... ... ... ... ... 148 



CHAPTER VIIL 



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 Blastula Germ-membrane, or Blastoderm. — Inversion (In- 

 vagination) 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 (segmentatio inequalis) and Hood- 

 gaetrula (Amphigastrula) of Amphibia and Mammalia. — Total and 

 Partial Cleavage. — Holoblastic and Meroblastic Eggs. — Discoidal 

 Cleavage (segmentatio discoidalis) and Disc-gastrula (Discogastrula) 

 of Fishes, Reptiles, Birds. — Superficial Cleavage (segmentatio super- 

 ficialis) and Vesicular Gastrula (Peri-Gastrula) of Articulates 

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

 Animals. — The Two-layered Primeeval Parent-form ; Gastraea. — 

 Homology of the Two Primary Germ-layers in all Intestinal 

 Animals (Metaz'u). — Significance of the Two Primary Germ- 

 layers. — Origin and Significance of the Four Secondary Gei 

 layers. — The Exoderm or Skin-layer gives rise to the Skin-sensor; 



