EMBRYOLOGY OF THALASSEMA MELLITA 227 



In Hydroides (Wilson, '91) there is a like close association 

 of the mesoblasts and, indeed, it is quite possible that in 

 all annelids with equal cleavage we have an identical origin 

 of the ectomesoblast, at least as far as the third quartet is 

 concerned. 



Embryologists during the last twenty-five years have been far 

 from unanimous in their conception either of the origin or of 

 the phylogenetic significance of the mesoblast. Hatschek 

 ('78) was among the first to lay emphasis on the differences 

 between mesenchyme and mesothelium, but his interpretation 

 of the embryology of Polygordiiis {'yS), Echuiyus ('81) and 

 Eupoinatiis ('85) led him to the conclusion that these two mor- 

 phologically different mesoblasts arise from a common founda- 

 tion. This view was adopted and developed by the Hertwig 

 brothers ('81) in their well-known *' Coelomtheorie," which, as 

 Meyer observes, has formed the foundation of all later work on 

 the mesoderm. Among other investigators who have described 

 the mesoblast as having a single origin are: Roule ('89, '94), 

 Fraipont ('88), Wilson ('89, '92), Hacker('95), Burger ('91, '94) 

 and Rabl ('89, '97). On the other hand, the great majority of 

 those who have studied the embryology of annelids and mol- 

 luscs are agreed that the mesenchyme must be regarded as hav- 

 ing an origin distinct from that of the coelothel. Although a 

 few have ascribed an endoblastic source to the former, the con- 

 census of opinion seems to be at present that it originates from 

 the ectoblast. This was first described to be the case by 

 Kleinenberg i^j'^, '86). Among those who have subsequently 

 confirmed his conclusions are : Whitman i^^'^7^, Berge ('90), 

 Vejdovsky ('90, '92), Schimkewitsch ('94), Meyer ('01), and the 

 extensive list of cytogenists that has already been given. 



Treadwell in a discussion of the phylogenetic significance of 

 the mesoblast takes the stand that *' no hard and fast distinction 

 can be made between the two forms of mesoblast." He bases 

 his conclusion on the close association of the coelomesoblast 

 and mesenchyme in Nereis, Liunbriais and Hydroides as de- 

 scribed by Wilson and also in Capitella (Eisig). This view is 

 diametrically opposed to that expressed by Meyer ('90, '01), as 



