CONTENTS. 



Y. Later Development. 



1. Closure of Blastopore 



2. Development of Larva 



68 

 71 



VI. Comparisons with A.mphioxus and Amphibia 73 



1. Axial Relations of Egg and Embryo ......... 75 



2. Entrance of Spermatozoon ........... 77 



3. Cleavage , . 77 



4. Blastula and Gastrula ........... 79 



5. Closure of Blastopore ............ 79 



6. Neural Plate 82 



7. Chorda 83 



8. Origin of Mesoderm ............ 84 



VII. The Organization of the Egg. 



A. Polarity 87 



B. Symmetry .............. 90 



C. Cytoplasmic Localization ...... .... 



1. Localization in the Cleavage Stages ........ 



93 

 93 

 95 

 99 



Cytoplasmic Organization and the Nuclear 



2. Localization before Cleavage 



D. Genesis of the Organization of the Egg 



1. Role of the Nucleus in Differentiation 



Inheritance Theory ... ........ 99 



2. Factors of Localization ........... 102 



a. Cytoplasmic Movements . . . . . . . . . .102 



b. Cell Division as a Factor of Localization 



E. Types of Germinal Localization ; Evolution of Types 



1. Annelid-Mollusk Type .... 



2. Ctenophore Type . . 



3. Echinoderm Type ..... 



4. Ascidian Type ...... 



103 

 104 

 105 



100 

 100 

 107 



Summary 



I. Ascidian Embryology 



II. Cytological Results 



III. Organization of the Egg 



Literature Cited 

 Explanation of Figures 



111 

 112 



11-' 



115 

 119 





