No. I.] THE EMBRYOLOGY OF CREPIDULA. 3 



PAGE 



a. Archenteron 157 



b. Intestine 157 



c. Stomach 159 



Comparisons 160 



VIII. Axial Relations of Egg and Embryo 163 



1. The Primary Cleavages 163 



2. Establishment of the Larval Axes 167 



3. Beginnings of Final Asymmetry 171 



D. General Considerations 173 



1. The Forms of Cleavage 17-j 



a. The Radial Type 174 



(i) Orthoradial Cleavage 174 



(2) Spiral Cleavage 17 c 



b. The Bilateral Type 183 



c. Significance of the Forms of Cleavage 185 



(i) Orthoradial 186 



(2) Spiral 187 



(3) Bilateral 189 



d. Determinate and Indeterminate Cleavage igo 



2. Cell and Regional Homologies 102 



a. Cell Homologies among Annelids and Mollusks 192 



b. Regional Homologies 201 



Conclusions 202 



References . 



205 



A. INTRODUCTION. 

 I . Purpose and History of the Work. 



The purpose of the following work from its inception has 

 been to make as careful a study as possible of the cleavage of 

 the ovum, the formation of the germinal layers and definitive 

 organs, and the axial relations of the ovum to the larval and 

 adult axes. At the time when this work was begun, several 

 years ago, scarcely any attempts had been made to trace the 

 history of individual blastomeres through the entire develop- 

 ment to the formation of definitive organs. The early stages 

 of cleavage had received a great deal of attention, but the 

 later stages had been largely neglected ; and although the 

 origin and homology of the germ layers was perhaps the most 

 frequently discussed subject in embryology, yet the relation of 

 these layers to the individual blastomeres of the cleaving ovum 

 had been determined in comparatively few cases. Since that 



