TKOCHOPHORE OF HYDEOIDES UNCINATUS (EUPOJVIATUS). 553 



in the course of time to certain cells in the wall of the archen- 

 teron, and as development became progressively specialised 

 their origin became localised in the posterior cell of the 

 fourth quartette. 



It is from the wider and more definite facts of comparative 

 anatomy rather than from those drawn from development 

 alone that the real value of Meyer's theory lies. It is evident 

 that the germ-cells — the foundation of the later coelomesoblast 

 — are much older in the phyletic sense, as Kleinenbergh (21) 

 and Meyer (27) long ago pointed out, and as Eisig (11) has 

 recently stated, than the two primary germ layers, and that 

 they were differentiated long before the ectoderm and endo- 

 derm had been evolved as separate structures, as is the case 

 to-day in Vol vox. Meyer's theory has been recently con- 

 sidered by Lang (24) and Eisig (11) so exhaustively that it is 

 unnecessary for me to go into it here. No theory certainly 

 accounts for so many facts or has been so widely supported 

 by evidence, both anatomical and enibryological. 



Thus the separation of the mesoderm into two definite 

 portions is a characteristic feature of the development of 

 Polyclads, Annelids, and Molluscs. It remains to be men- 

 tioned that in a few Molluscs a larval or ecto-mesoblast has 

 not been observed or is apparently wanting. This would 

 seem to be definitely the case in Aplysia, the late stages of 

 which have recently been studied by Carr Saunders and Miss 

 Poole (31). In Umbrella, Heymons (20) has been unable 

 to find this structure, but he suggests that possibly in stages 

 later than those he studied ectoderm cells may migrate into 

 the interior of the larva and form mesoderm. In Neritina 

 Bloclnna,iin (4) also fails to figure it; but both Heymons and 

 Bloc•hmann^s work was done at a time when the importance 

 of the larval mesoderm was hardly recognised, and ultimately 

 it may prove to be present in these forms. Its absence in 

 Aplysia, however, seems to be clearly established. It is 

 hard to understand why this should be the case, as the 

 majority of Molluscs possess a larval mesoblast, and one is 

 present in Fiona. 



