THE EARLY DEVELOPMENT OF PLANORBIS. 



SAMUEL J. HOLMES. 



CONTENTS. 



Part I. Descriptive Portion. ^^^ 



Methods 370 



Nomenclature 373 



The Eggs and Egg Masses 373 



The First Cleavage 375 



The Second Cleavage 376 



The Third Cleavage 378 



From the Eight to the Twenty-four Cell Stage 379 



Cleavage Cavity and Vacuoles 385 



From the Twenty-four to the Forty-nine Cell Stage 3S6 



The Division of the Second Quartette and the Formation of the Cross 386 



The Trochoblasts 387 



The Division of the Third Quartette 390 



The Formation of the Primary Mesoblast 391 



The Fourth Quartette 393 



The History of the Cross .« 393 



The Second Quartette 403 



The Mesoblastic Bands 407 



The Third Quartette and the Secondary Mesoblast 408 



General Considerations on the Secondary Mesoblast 411 



The Entomeres 413 



The Rudiments of the Cerebral Ganglia and Eyes 415 



The Apical Plate 416 



The Cell Lineage of the Head Vesicle 417 



The Cell Lineage of the Prototroch 418 



The Shell Gland and the Foot 421 



The Larval Kidney 422 



Gastrulation ; Fate of the Blastopore 425 



Part II. General Considerations. 



Reversal of Cleavage and Reversed Asymmetry 426 



The Relation between Reversed Cleavage and the Direction of the First 



Cleavage Plane 429 



General Considerations on Spiral Cleavage 430 



Reversal of Cleavage and Cell Homologies 440 



Bibliography 445 



Explanation of the Figures 450 



369 



