1904. ] NATURAL SCIENCES OF PHILADELPHIA. 325 
THE CELL-LINEAGE AND EARLY LARVAL DEVELOPMENT OF FIONA 
MARINA, A NUDIBRANCH MOLLUSK,Y 
E 
BY DANA BRACKENRIDGE ’CASTEEL, PH.D. 
OUTLINE. 
Introduction. 
Material and Method. 
Nomenclature. 
Earlier Work on Opisthobranch Development. 
Maturation and Fertilization. 
The Unsegmented Egg. 
First Cleavage. 
Second Cleavage. 
Origin of Germ Layers. 
Segregation of the Ectoblast. 
Segregation of the Ento-Mesoblast. 
Segmentation of the Entoblast. 
Cleavage History of the Ectomeres. 
The First Quartet. 
The Second Quartet. 
The Third Quartet. 
Gastrulation. 
Ecto-Mesoblast 
Closure of the Blastopore. 
Organogeny. 
The Velum. 
Later Velar Development. 
Head Vesicle. 
Nerve and Sense Organs. 
Cerebral Ganglia. 
Otocysts and Pedal Ganglia. 
Eyes. 
Excretory Organs. 
The Enteron. 
Stomodzum and Mouth, 
Shell Gland and Foot. 
Larval Musculature. 
Change of Axis and Form of the Developing Organism. 
Abstract. 
Table of Cell-Lineage. 
INTRODUCTION. 
The study of the cleavage and early larval history of Fiona marina 
(Forsk.)? embodied in this paper was undertaken with the view of 

1 Contribution from the Zoélogical Laboratory of the University of Penn- 
sylvania. 
“2 Dr. H. A. Pilsbry, of the Academy of Natural Sciences of Philadelphia, has 
kindly assisted me in identification. 
