247 



Nachdruck verboten. 



The Early Development of the Mesohlast in Thalassema. 



By John Cutler Tokrey. 

 With 3 Figures. 



The early development of Thalassema throws some interesting 

 light on the question of the origin of the mesoblast, especially when 

 brought into comparison with Meyer's^) observations on its two-fold 

 origin in the larval stages of annelids, and also with the results of 

 students of cell lineage who have found a "larval mesenchyme" arising 

 from the ectoderm and entirely independent of the "primary mesoblast" 

 or pole cells of the "mesoblast-bands". An investigation of the cyto- 

 geny of Thalassema mellita (Conn) not only clearly demonstrates the 

 two-fold origin of the mesoblast, but also that the ectomesoblast -) here 

 arises from all of the first three quarters of ectomeres instead of from 

 one alone as has hitherto been described to be the case. A con- 

 siderable number of these cells, however, are rudimen- 

 tary and quickly disappear. 



It has been my aim not only to trace the various sources of the 

 ectomesoblast, but, as far as possible, the ultimate fate of each element 

 in accordance with Ray Lankester's observation that "we cannot get 

 further with the analysis of mesenchyme until the first origin and sub- 

 sequent history of every constituent cell in a series of typical examples 

 has been determined"'^). The cleavage of Thalassema is especially 

 favorable for such a study as there is very little yolk and the cleav- 

 age cavity is from the first large. 



Thalassema belongs to that rather limited class of annelids with 

 "equal cleavage" of which the best known representatives are Hyd- 



1) Meyer, E., Studien über den Körperbau der Anneliden. Mitth. 

 Zool. Stat. Neapel, Bd. 8, 14. 



2) The terms "ectomesoblast" and "entomesoblast" are used as con- 

 venient descriptive terms to emphasize the difference in origin of the 

 two forms of mesoblast in accordance with the suggestion made by 

 Professor E. B. Wilson in his paper on Cell Lineage and Ancestral 

 Reminiscence. Ann. N.-Y. Acad. Sc, Vol. 9, No. 1. 



The term ectomesoblast seems preferable to "larval mesenchyme" 

 now that Meyer has discovered that a good deal of mesoderm derived from 

 the ectoderm persists in the adult as circular muscles, gut muscles etc. 



3) Lankester, E. Ray, Treatise on Zoology, Vol. 2, p. 31. 



