166 TORREY 



Introduction. 



It has been the primary object of this investigation ^to trace 

 as fully as possible the history of the mesoblast, and especially 

 of the ectomesoblast ^ (often called ** mesenchyme") in TJialas- 

 senia,- of which I have endeavored to determine not only the 

 origin, but also the fate of every constituent cell. In some ways 

 this form has proved extremely favorable for such a research, 

 since the cleavage-cavity is from the first large, the ectomeso- 

 blast abundant, and the coelomesoblast very late in developing. 



As has been indicated recently by Ray Lankester,'^ the study 

 of cell lineage has proved and will continue to prove an effi- 

 cient aid in the solution of the mesoblast question. Not alone 

 in this problem, but in general, I believe, one of the principal 

 values of this method of study lies in the fact that it supplies 

 the firmest possible foundation for the investigation of all later 

 morphogenic processes, whether normal or experimentally 

 modified. If so much had not been taken for granted regard- 

 ing the cleavage stages, many embryological blunders might 

 have been avoided. But it is equally true that if many of the 

 earlier embryologists began their studies at too late a period in 

 the development, a number of the later students of cell-lineage 

 have stopped too soon and have failed to connect definitely the 

 various cell regions that they have so carefully described, with 



^Throughout the paper the term " ectomesoblast " (Wilson, '98) is used in 

 preference to the term " larval mesenchyme" (which has been employed by the 

 majority of the students of cell-lineage), or the term " psedomesoblast " (Eisig, 

 '98). I think this preference is warranted, first, by its origin in Annelids and 

 Molluscs from cells of the ectoblastic quartets, and second, by Meyer's discovery 

 that a large part of this mesoblast persists in the adult in the form of circular 

 muscles, gut-muscles, etc. This is also in all probability the case in T/ialassenia. 

 The term " coelomesoblast" (Eisig) is employed to designate the mesoblast which 

 arises from the posterior member of the fourth quartet {^d) and produces the sec- 

 ondary body cavity (*' entomesoblast " of Wilson). 



^The only existing account of the development of this genus (with the exception 

 of a brief communication by Kowalevsky, '72) is the one given by Conn in 1886. 

 This account covers the entire history, but, as might be expected in so early a paper, 

 the early stages are treated very superficially and many of his descriptions are radi- 

 cally wrong. 



^Lankester, E. Ray, "Treatise on Zoology," Vol. 2, Introduction. 



