CONTENTS 

 Preface page v 



PART ONE 

 THE FACTS OF DEVELOPMENT 



I. THE SCIENCE OF EMBRYOLOGY 3 



1. The place of embryology among the biological sciences 3 



2. An outline of development 5 



3. Phylogenetic theories of embryology 8 



4. The mechanisms of development 10 

 Appendix. The concept of embryonic fields 23 



n. THE GAMETES 29 



1. Spermatogenesis 30 



2. Oogenesis 31 



3. Follicles and membranes 37 



4. TAe morphogenetic structure of the egg 40 



m. FERTILISATION 45 



1. Activation 47 



2. T/ie i»i/o« (>/"f/2e nuclei 50 



3. Artificial parthenogenesis 51 



IV. CLEAVAGE 57 



1. General features 57 



2. TAe pattern of cleavage and the pattern of the embryo 62 



3. Differentiation ivithout cleavage 63 



4. Cleavage without nuclei 64 



5. The mechanism of cleavage 64 



V. THE ECHINODERMS ' 79 



1. Normal development 79 



2. T/?e gradient system 82 



3 . The dorso-ventral axis 92 



VI. SPIRALLY CLEAVING EGGS 94 



Vn. THE ASCIDIANS AND AMPHIOXUS IO5 



