136 TI;AN.SA.0TI0NS and PJtOCEKDINGS OF THE [Sess. Lxvi. 



more recent results must be taken together, for Wilson 

 himself has seen reason to alter his earlier interpretation 

 in some slight but important respects. These amendments 

 are exactly of the kind required for my reading of his 

 table of the cell-lineage. 



In Fig. 2. Wilson's table, so far as it concerns us, is 

 reproduced, and in Fig. 3 the same results are shown after 

 the manner of Boveri's figure, or the part of my diagram 

 from Z to U.K.Z. 



The main difficulty of the writer in his reading of 

 Wilson's diagram has hitherto been the supposed destiny 

 of the cell d'^ — x. From p. 30 of his recent lecture it 

 may be gathered that the author, following the iinds of 

 Lillie in Uaio now looks upon this cell x as representing a 

 larval mesoderm-cell. This is exactly the fate it ought to 

 have, for, as we have seen, everything to the left of the 

 line Z to U.K.Z. must belong to the phorozoon or larva. 



The primitive germ-cell has not yet been identified in 

 Annelida. From my interpretation of Wilson's finds it 

 would appear to arise in Nereis at the fifth cleavage as 

 the cell in Fig. 3 labelled D - U.K.Z. This then divides 

 into (two primary germ-cells) D and M. M is the 

 primary mesoderm-cell or somatoblast of various authors. 

 Its division initiates the period of bilateral cleavage. Its 

 two products form the two " mesoderm-bands." In con- 

 tradistinction to other authors, the writer must maintain 

 the opinion that M is a primary germ-cell, and that it 

 gives rise to the whole of the sexual generation, in this 

 case the worm. In this connection it may be of interest 

 to recall the circumstance that long ago Hatschek expressed 

 the view that the two products of M, the well-known " pole- 

 cells " of Hatschek, were originally eggs.^ This suggestion 

 has been criticised by Kleinenberg. 



If the pole-mesoderm-cells be not eggs, they at least 

 arise by the division of the next thing to an egg, a primary 

 germ-cell. 



In Nereis the remaining primary germ-cell D comes to 



1 Wilson approves of E. Meyer's amendment of Hatschek's view into 

 a correspondence of the mesodermal bands with paired gonads. In 

 the sense indicated above, there is much to be said in favour of 

 Hatschek's interpretation ; the other idea is wildly impossible. 



