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I. INTRODUCTION. 



The peculiar embryonal envelope of Sipunculus nudus, with its associated amni- 

 otic cavities described by Hatschek ('83), has hitherto been regarded as a structure 

 sui generis. 



Hatschek ('80) discovered nothing of a similar nature in Echiurus; and yet among 

 the annelids the Echiuridae may perhaps be regarded as the nearest allies of the sipun- 

 culids. Nor did the observations of Selenka (75) upon Phascolosoma throw any 

 light upon this remarkable feature in the development of Sipunculus. 



So little has been known of the embryology of this interesting group that at the 

 suggestion of my friend Dr. C. A. Kofoid I undertook the study of the development 

 of Phascolosoma gouldii Diesing. My work was begun in the summer of 1893 at 

 the laboratory of Dr. Alexander Agassiz at Newport, R. I. Observations made at 

 Newport proved this locality to be so favorable for my work that on the succeeding 

 year by the kindness of Mr. Agassiz I continued my studies there, and was able to 

 follow the development of the trochophore and larva until the latter had reached the 

 age of thirty days. My first attempts to study the cleavage stages, while still a student 

 under Dr. Mark at Harvard University, met with only a partial success, owing to the 

 difficulty presented by the thick and highly refractive yolk -membrane (zona radiata) 

 to staining and preparing in balsam the somewhat opaque eggs and embryos of this 

 species. After repeated attempts at Wood's Hole, Mass., during the summers of 1896 

 and 1897 to obtain material for this study, I went to the Laboratoire Lacaze-Duthiers 

 at Roscoff in Finistere, where I enjoyed the hospitality of the founder. There in the 

 summers of 1898 and 1899 I was able to work out somewhat in detail the cleavage of 

 the beautifully transparent egg of Phascolosoma vulgare Blainville, to compare the 

 arvae of this species with those of P. gouldii, and to collect material for further 

 investigation. The work was extended by studies in P. gouldii at Wood's Hole 

 during the summers of 1900 and 1902. 



I have made several attempts to fertilize the eggs of Sipunculus, once on October 1 

 at the laboratory of the College de France at Concarneau in lower Brittany, once in 

 July at Roscoff with specimens collected at Trez Hir near Brest, and several times 

 during the winter months at Naples. There is reason to believe that renewed efforts 

 at Concarneau and Trez Hir, where Sipunculus is abundant, or possibly at Naples, 



