EMBRYOLOGY OF THALASSEMA MELLITA 167 



the organs of the adult or even of the larva. This is especially 

 true of the so-called " larval mesenchyme " (ectomesoblast). 

 The cytogeny of a number of annelids and molluscs has shown 

 that the mesenchyme arises by the insinking or delamination of 

 ectomeres from either the second or third quartet, but never 

 from more than one. In TJialassema, however, not only does 

 functional " mesenchyme " arise from the same cells in the third 

 quartet as in Podarke (Treadwell, 'oi), but also from a number 

 of cells in the first quartet. Under the category of ectomeso- 

 blast I think may also be placed certain cells from the second 

 and first quartets, which appear to have completely lost their 

 function. These rudimentary cells sink into the segmentation- 

 cavity, and are finally ingested and completely absorbed by the 

 entoblast cells. It is a significant fact that these cells, with but 

 two exceptions, have a radial arrangement and these two excep- 

 tions are found where the bilaterality of the embryo is first fore- 

 shadowed, /. e., in the posterior arms of the praetrochal group 

 of cells forming the structure designated by cytogenists as the 

 " cross." A very probable explanation of such cells seems to 

 be that they are vestigial structures, ancestrally reminiscent of 

 certain radial and probably mesenchymatous organs in the 

 ancestor of the annelids. All these facts tend to sustain the 

 view that the mesoblast Jiad primitively a radial origin (Conklin, 

 '97) ; and also that primitively the mesenchyme arose from all 

 three ectodermal quartets (Wilson, '97). 



One of the most interesting questions in regard to the meso- 

 blast, on which these observations bear, is whether we may, or 

 may not, regard it as a morphological unit. Possibly the in- 

 vestigation has not been carried far enough in Thalasscma to 

 decide the question definitely for this form, but it is evident that 

 in origin, at least, and I think without doubt in fate also, we 

 are dealing with two distinct forms of mesoblast ; the one aris- 

 ing from the posterior member of the fourth quartet and pro- 

 ducing the secondary body-cavity, and the other by the insink- 

 ing of certain ectomeres and not only producing all the mesen- 

 chyme of the larva, but also contributing to the mesoblast of 

 the adult. 



