TROCHOPHORE OF HYDROIDES UNCINATUS (eUPOMATUs), 547 



before the mesoblast bauds. They are ah-eady fnuctioual 

 befoi"e the bands have appeared, for the pole-cells so con- 

 spicuous in Hatschek^s figures liave no existence at this 

 stage. 



It is true that Meyer (27), from tlie study of late stages, 

 came to the conclusion that larval mesoblast was a structure 

 apart from the coelomesoblast, but I doubt if the evidence 

 derived from the study of tliese late stages in Polygordius 

 alone is sufficiently convincing. 



Text-fig. 6 a. 

 PocLa^rke 



{rr-- 



Section tlirongli a late gastrula stage of Podarke (Treadwell). 

 a. Archenteron. (t.r. Apical rosette. e. First endodevni 

 cell budded off from 4d.y l.ni. Larval or ectomesobhist. 

 ME. Cceloinesoljlast. 



Within the last twenty-five years a large literature has 

 grown u}) with regard to the question of the mesoderm, and 

 embryologists have held many opinions regarding its origin 

 and significance. These conflicting views are roughly 

 reducible, howevei-, to two groups, each of which has been 

 advocated with more or less success. To the first belong 

 those who consider the mesenchyme (larval mesoblast, 

 ectomesoblast) and me sot helium (definite mesoderm or 

 coelomesoblast) as one and the same structure ; to the second 

 belong those who consider them as two separate structures. 



