EMBRYOLOGY OF THALASSEMA MELLITA 229 



possible that he has overlooked the true origin of the coelomes- 

 oblast and that what he describes as such is, in fact, the mesen- 

 chyme, especially as its origin is very similar to that of Thal- 

 assema. Over against this case we may place thirty or more 

 forms of annelids, where the coelomesoblast has been shown to 

 arise from the posterior member of the fourth quartet. I be- 

 lieve that Treadwell has not sufficiently considered the problem 

 in concluding that because the trochophore may '* represent an 

 ancestral stage in the phylogeny of annelids, the mesodermal 

 structures found in it undoubtedly represent the mesoderm of 

 the ancestral forms." 



Meyer, in his last paper ('oi), has given a most exhaustive 

 review of the whole mesoderm question and has shown how 

 strongly the great mass of the evidence, both embryological 

 and anatomical, points to the conclusion that, in annelids, at 

 least, there are two entirely distinct forms of mesoblast — the 

 primary (ectomesoblast) and the secondary (coelomesoblast). Of 

 these he considers the primary mesoblast to be phylogenetically 

 the older and, although as a rule derived from the ectoderm 

 (Phillodocidae, Aphroditidae, Eunicidae, Chaetopteridae), " cannot 

 be considered a morphological unit, but is rather an embryologi- 

 cal synthetic tissue, in which temporarily the undifferentiated 

 foundations of very diverse organs and tissues are apparently 

 united to a whole." The coelomesoblast, on the other hand, is 

 regarded as a later formation which has originated from gonad 

 cells. If we add to Meyer's observations the results of the 

 study of cell-lineage, his conclusions, I think, are greatly strength- 

 ened. New light will certainly be thrown on this question by 

 a study of the cytogeny of some of the lower forms. In this 

 connection the observations of Schimkewitsch on the develop- 

 ment oi Dinophiliis are, to say the least, very suggestive. He 

 found in this type two forms of mesoblasts, distinct in origin, 

 one originating at the posterior end of the embryo and forming 

 two bands, which later give rise to the gonads and possibly to 

 certain ventral longitudinal muscles, the other consisting of 

 scattered cells which have migrated into the cleavage cavity 

 from the anterior end of the embryo and produce all the mesen- 



