CONTENTS. IX 



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

 between the Processes of Individual and of Kace 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 Eeproduction. — Heredity.— Division of Labour, 

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

 Functions of Evolution as yet very lictle studied by Physiology, 

 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 Egg-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. — Monernla. — Reversion to the Monera- 

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



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 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 ; Exoderra and Entoderm. — Kenogenetic Form of Egg- 

 cleavage. — Unequal Cleavage (segynentatio inequalis) and Hood- 

 gastrula {Am^M gastrula) 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 {Feri-Gastrula) of Articulates 

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

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

 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 



