6 CONK LIN. [Vol. XIII. 



This work was begun in the summer of 1890, while I was 

 occupying the Johns Hopkins University table at the Marine 

 Laboratory of the United States Fish Commission at Wood's 

 Holl, Mass. During the succeeding winter I continued the 

 work in Professor Brooks' laboratory at Baltimore, and in the 

 summer of 1891 I again occupied the Johns Hopkins table at 

 Wood's Holl, and continued to work on the same subject. 

 Since that time my work has suffered long and repeated inter- 

 ruptions owing to the pressure of other duties. 



I had hoped to be able to present in one paper both the 

 earlier and the later stages in the development, but the work 

 has grown so much, both in extent and difficulties, that it has 

 seemed best to publish the results of investigations on the 

 early stages first, and to supplement these by another paper 

 on the later stages as soon as possible. Since the study of the 

 later stages is less general in its bearing and more specifically 

 applicable to the Mollusca, such a division of the subject will 

 not be an illogical nor an unwelcome one. Two preliminary 

 papers have been published on this subject, — one on the gen- 

 eral embryology of Crepidula and Urosalpinx (Conklin, '91), the 

 other on the cleavage in Crepidula ('92). 



During the first year of the work my attention was directed 

 exclusively to the development of Crepidula fornicata, and a 

 large number of drawings of the various stages in the embry- 

 ology of this species were made ; for this reason it forms the 

 chief subject of this paper, although in some respects C. plana 

 is a more favorable object for study. It was not until the sum- 

 mer of 1892 when, through the courtesy of Professor Whitman, I 

 was enjoying the privileges of the Marine Biological Laboratory 

 at Wood's Holl, that I obtained material for the study of the 

 embryology of C. plana and C. convexa. I have, however, 

 made a careful comparison of the development of these three 

 species, and in most respects have found the cleavage and 

 formation of the germ layers and larval organs very similar in 

 all of them. 



Through the kindness of my friend and former pupil, Mr. 

 Harold Heath of the Leland Stanford University, I have 

 recently received a number of adult specimens and a good col- 



