548 CUKSSWELL SHEARER. 



The first consider they have a commou, while the second 

 claim they have a separate origin. 



Hatschek, as the result of his studies on Polygordius (18), 

 Echiurus (16), Eupomatus, and Teredo (17 and 15), 

 many years ago pointed out the difference between the 

 irregular scattered cells of the mesenchyme and the deHnite 

 cells of the mesoblast bands. He claimed, however, to have 

 observed the oi-igin of the mesenchyme cells from the meso- 

 blast bands. In his opinion mesenchyme and mesothelium 

 arise from a common foundation. This was folloAved by 

 Wilson^s (48 and 47) work on Hydroides, Polygordius 

 and Lumbricus, where he found a complete gradation from 

 the stellate cells of the mesenchyme scattered through the 

 blastocoel to the round fixed cells of the anterior ends of the 

 germ bands. Many other observers have pointed out more 

 or less the same thing, as, for instance, Roule (29) in Jl^uchy- 

 trffioides, Fraipont (12) in Polygordius, Burger (5) in 

 Nephelis, Hirudo and An las torn a, Hacker (13) in 

 Polynoe. The common nature of both mesenchyme and 

 mesoderm at one time gained wide acceptance through its 

 adoption and elaboration by the brothers Hertwig (19) in 

 their well-known ' Coelomtheorie.' 



On the other hand, the majority of those embrvologists 

 who have recently investigated the development of Annelids 

 and Molluscs hold that these structures are both ontologically 

 and ])hylogenetically distinct ; that the mesenchyme has an 

 origin apart from the coelomesoblast, that it arises in a 

 peculiar fashion from the ectoderm ; hence they have sought 

 to denote this in the name they have applied to it, i.e. that 

 of ectomesoblast. The coelomesoblast, on the contrary, is 

 usually segregated in a single large cell seen in the ventral 

 lijs of the blastophore. 



Kieinenbergh (21) was perhaps the first to lead the way 

 towards this conception of the nature of mesoderm and 

 mesenchyme, in his paper on the development of Lopado- 

 rhynchus, where he pointed out that the mesoderni arises 

 as a membrane between the two prinrary layers, and, ns ];e 



