584 GRIFFIN. [Vol. XV. 



INTRODUCTION. 



In the autumn of 1895, Prof. E. B. Wilson placed in my 

 hands a very complete series of maturation and fertilization 

 stages of the echiuroid Thalassema mellita Conn, collected by 

 him at Beaufort, N. C, for a careful study of the achromatic 

 structures with especial reference to the structure and function 

 of the centrosome in maturation and fertilization. The research 

 was at first exclusively confined to Thalassema, which proved 

 an extremely favorable object. During the following summer, 

 however, it was the writer's good fortune to be able to collect 

 a series of stages of the large piddock, or "boring clam," Zir- 

 phaea crispata from the Pacific shores of the United States. ^ 

 As the maturation and fertilization phenomena in the Lamelli- 

 branch also were at that time wholly unknown, the investiga- 

 tion of Zirpliaea was immediately undertaken in connection 

 with further study of Tlialasscma. 



The work upon the achromatic elements produced results so 

 clear and convincing that the writer was encouraged to take up 

 a study of the chromatin with reference to the reduction ques- 

 tion, which was then, as now, in fully as unsettled a condition 

 as that respecting the centrosome. It was in this portion of 

 the research that Zirpliaea yielded the most important evidence, 

 furnishing in fact the key to the understanding of the process 

 in Thalassema. 



dissertation for the degree of Doctor of Philosophy just before his last illness. 

 Besides slight verbal alterations, which have not modified in any essential way 

 either the substance or the form of his conclusions, the only changes that have 

 been made are omissions of detail and the insertion of references to the figures. 

 No literature-list accompanied the manuscript, and it has seemed best not to 

 attempt the preparation of one. The text-figures have been prepared from 

 sketches made as memoranda on the margin of the manuscript; but the author's 

 methods of work were so conscientious that their accuracy may safely be assumed. 

 An earlier and briefer paper on Thalassema, containing some details not included 

 in the present work, was published by Mr. Griffin in the Transactions N. Y. Acad. 

 Sci., vol. XV, June, 1896, under the title "The History of the Achromatic Struc- 

 tures in the Maturation and Fertilization of Thalassema." I am indebted to 

 Mr. Henry E. Crampton, Jr., for revision of the proofs of the present paper. — 

 Edmund B. Wilson. 



' Cf. Harrington and Griffin on Distribution and Habits of Some Puget 

 Sound Invertebrates," Transactions of N. Y. Acad. Sci., March, 1S97. 



