62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



I. Introduction. 



In the inception of this work on the barnacles of the genus Lepas it 

 was planned to make a careful investigation of the early development 

 with reference to the origin and fate of the germ-layers. With this 

 object in view the methods of workers on cell-lineage were adopted, 

 because detailed studies seemed necessary in order to determine accu- 

 rately the origin of the germ-layers. These studies were not undertaken 

 with any expectation of extending or testing the accuracy of the generali- 

 zations which have come from the epoch-making investigations on cell- 

 lineage in the eggs of annelids, mollusks, and other animals. Whatever 

 opinion may be held regarding the fundamental importance of the gen- 

 eralizations growing out of such studies, it is usually conceded that the 

 tracing of cell-lineage gives a basis for accurate description of the details 

 of embryological development. Such accuracy in itself seems to furnish 

 sufficient present justification for studies in cell-lineage, for no one can 

 predict what interpretations may in the future grow out of any recorded 

 facts of to-day. 



A study of Lepas fascicularis was begun by me in June 1894. Late 

 in that year there appeared an elaborate and important paper by T. T. 

 Groom on the development of several Cirripedia. As stated in a prelim- 

 inary note (Bigelow, '96), my independent studies of Lepas fascicularis 

 partly confirmed Groom's results in the case of other species of tjiis 

 genus, but evidence in hand at the time of the publication of Groom's 

 paper indicated that, so far as accurate description of cleavage and the 

 formation of germ-layers is concerned, his account did not agree with 

 the development as observed in L. fascicularis. The studies already 

 begun by me were, therefore, continued and extended to Lepas anatifera 

 and other species which Groom had described. The account given in 

 this paper is based primarily upon studies of L. anatifera, and L. fasci- 

 cularis. 



I take this opportunity to express my great indebtedness to my former 

 teacher, Prof. E. G. Conklin of the University of Pennsylvania, under 

 whose guidance the general outlines of the work were developed. 



The completion of the observational work was carried out during the 

 year 1898-99 in the Zoological Laboratory of the Museum of Compara- 

 tive Zoology at Harvard College. To all the instructors of the depart- 

 ment I am greatly indebted for stimulating interest, but especially do 

 I owe acknowledgment to Dr. W. E. Castle, who continuously followed 

 my work and gave me the benefit of his advice and criticism, and to 



