Volume X. January, iSgS- Numberl. 



JOURNAL 



MORPHOLOGY, 



THE EMBRYOLOGY OF THE UNIONIDAE. 



A STUDY IN CELL-LINEAGE. 

 FRANK R. LILLIE. 



TABLE OF CONTENTS. 



I. Introduction 



II. Cleavage of the Ovum and Derivation of the Germinal Layers 



a. Natural History 5 



Methods 



Nomenclature 



The First Two Furrows ^o 



2. The First Generation of Ectomeres 13 



3. From the Eight to the Seventeen-cell Stage 18 



4. From the Seventeen to the Thirty-eight-cell Stage ; Third 



Generation of Ectomeres, etc ^9 



5. Establishment of Bilateral Symmetry 23 



6. The First Somatoblast ^* 



7. The Second Somatoblast ^7 



e. Gastrulation and Shell-gland 28 



f. Summary ^ 



g. General Remarks on the Cleavage and Germ-layers 32 



