CONTENTS. IX 



PAGE 



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 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 ; Exoderm and Entoderm. Kenogenetic Form of Egg- 

 cleavage. Unequal Cleavage (segmentatio inequalis) and Hood- 

 gastrula (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 swper- 

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

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

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

 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 



