326 PROCEEDINGS OF THE ACADEMY OF [April, 
obtaining, as far as possible, an exact knowledge of the development 
of this Opisthobranch, in order that certain doubtful points regarding 
the embryology of Mollusks in general, and this group in particular, 
might be better understood. Fiona has proved in many ways a diffi- 
cult object for study, but in certain respects offers advantages to the 
investigator. The exact origin of the germ layers as they arise in the 
segmenting egg has been particularly sought throughout the cleavage 
history, while in later stages attention has been directed to the rise of 
larval organs from their particular protoblasts where these could be 
definitely determined. Where this has been found impossible, approxi- 
mate results are given. Certain questions have presented themselves 
both at the beginning and during the progress of this work, some of 
which may here be indicated briefly. Though it has not been my pur- 
pose to consider particularly the mechanics of cleavage, this phase 
of development has been borne in mind, and in certain instances 
discussed. Comparisons are made between the nearly equal cleavage 
of Fiona and the more unequal segmentation of many other molluscan 
and annelidan eggs. The manner of origin of the germ layers is nat- 
urally a point of cardinal interest to the cell-lineage worker, since by 
this method of investigation the most exact results are possible and 
very definite comparisons with other forms may be made. The exact 
derivation of the middle germ layer has been sought particularly. 
Has it a single or double mode of origin? If both “primary” and 
“secondary”? mesoderm be present, which is “larval” and which forms 
permanent organs? How is the mesoderm segregated from the two 
primary germ layers? In the study of larval structure and develop- 
ment the excretory organs are of much interest, since widely diverse 
views are held regarding the mode of origin and the significance of 
both primitive and definitive molluscan kidneys. The axial relations 
between ovum and larva and the relations of the early cleavage planes 
to the median plane of the larva and adult are points of great interest. 
How and when does bilaterality first appear? When does trosion 
first become manifest and what is its immediate cause? These and 
other questions have arisen and have been borne in mind during the 
progress of the work. Unfortunately material for the study of later 
larval stages and metamorphosis has not been obtainable, so that a 
complete record of development from ovum to adult has been impos- 
sible. 
The work was begun in the early summer of 1901, at the Zodlogical 
Laboratory of the University of Pennsylvania, and continued, together 
with general graduate study, during the two following years at this 
