568 HAROLD HEATH, 



V. History of the First Quartette. 



1. Trochoblasts and Tip Cells, Formation of the Prototroch. 

 Comparisons. 



2. Ectoblastic Cross. (Molluscan Cross.) 



a) Its formation. 



b) Axial relations. 



c) Its occurrence in other forms. 



d) Later history. Comparisons. 



3. Eosette Series ("Annelid Cross"). 

 VI. History of the Second Quartette. 



1. Early Cleavages and Later History, Comparisons. 



2. First Somatoblast. Comparisons. 

 VII. History of the Third Quartette. 



1. Early Cleavages and Later History. Comparisons. 

 VIII. An account of the Invagination with Comparisons. 

 IX. History of the Mesomeres. 



1. The Mesoblast. 



X. History of the Entomeres. 

 • 1. The Fourth Quartette. 



2. The Fifth Quartette. 

 XI. The Free-Swimming Larva. 



1. Habits. 



2. Metamorphosis. 



3. The Foot. 



4. The Shell. 



XII. General Considerations. 



1. Cell Homologies among Annelids and Molluscs. 



2. The Ancestral Form of the Trochophore. 



3. Forms of Cleavage. 



4. Anachronism of Cleavage. 



5. Relationship of the Annelid and Chiton Trochophore. 



6. Metamerism. 



The following paper on the embryology of Ischnochiton was under- 

 taken primarily with the view of determining as far as possible the 

 accurate origin and development of the germ layers. As a result of 

 such a study it was hoped some light might be thrown on the phylo- 

 geny of the Mollusca, more especially with regard to the relationship 

 between this group and the Annelids. The work has expanded beyond 

 my expectations, and owing to this fact I have, in the following pages, 

 paid particular attention to the cell lineage, and the external features 

 of the development through the formation of the trochophore and its 

 metamorphosis, up to the assumption of the adult form. The deve- 

 lopment of internal organs , only in so far as it bears directly upon 

 the complete understanding of external structures, has been considered. 



