TABLE OF CONTENTS. xi 



PAGE 



Division 168 



Budding i6 9 



b. Sexual Reproduction: Amphigony 170 



Amphigony Defined 170 



Parthenogenesis and Paedogenesis 170 



Parthenogenesis and Typical Amphigony 170 



Sexual and Somatic Cells 171 



c. Combined Modes of Reproduction 172 



Occurrence in the Same Species 172 



Progressive Alternation of Generations: Metagenesis . . 172 



Heterogony .... 174 



Asexual and Sexual Reproduction ......... 174 



General Phenomena of Sexual Reproduction 175 



1. Maturation 175 



Formation of the Polar Bodies 176 



Relation between Maturation and Fertilization .... 177 



2. Fertilization, Copulation, and Artificial Fecundation. . 177 



Fertilization 178 



Monospermy and Polyspermy 178 



Essential Feature in Fertilization 179 



Part Played by the Two Nuclei in Fertilization .... 180 



Heredity 181 



The Material Basis of Heredity iSi 



3. Cleavage Process 182 



Arrangement of the Cleavage-planes 182 



Influence of the Yolk upon the Cleavage Process . . . i ; 2 



Distribution of the Yolk 183 



Various Modes of Cleavage 184 



a. Holoblastic Eggs with Total Cleavage 185 



b. Meroblastic Eggs with Partial Cleavage .... 185 



Distribution of the Modes of Cleavage ....... 186 



Blastula 187 



4. Formation of the Germ-layers 187 



Gastrula 187 



Invagination iS8 



Modified Modes of Gastrulation iSS 



Epiblast and Hypoblast iSS 



Delamination iSS 



Formation of the Mesoblast. The Mesenchyme . . . 189 



Mesepithelium 190 



Ccelomic Pouches 191 



Occurrence of Mesenchyme and Mesepithelium . . . 191 



Histological and Organological Differentiation .... 191 



Relations of the Germ-layers in Budding 192 



5. The Different Forms of Sexual Development .... 192 

 Embryonic and Post-embryonic Development .... 192 



