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 nearl}^ 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 ■s\'ith 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? Wlien does t^sion 

 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 imiDOs- 

 sible. 



The w^ork was begun in the early summer of 1901, at the Zoological 

 Laboratory of the University of Pennsylvania, and continued, together 

 with general graduate study, during the two following years at this 



