354 Beard, Heredity aud the epicycle of the germ-cells. 



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 Bo- 

 veri's figure, or the part of my diagram from Z to U. K. Z. 



The main difficulty to the writer in his reading of Wilson's 

 diagram has hitherto been the supposed destiny of the cell d2 = x. 

 From p 30 of his recent lecture it may be gathered, that the author, 

 following the finds of Lillie in Unio, now looks upon this cell x as 

 representing a larval mesoderna-cell. This is exactly the fate it ought 

 to have; for, as we have seen, everything to the left of the line Z 

 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. 



Fig. 2. 



r f r 



/ / / 



OV. CD D D 



"UKi 



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 contradistinction 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 

 ofM, the well-known ,,pole-cells" of Hatschek, were originally eggs 1 ). 

 This suggestion has been criticised by Kleineuberg. 



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

 into correspondence of the mesodermal bands with paired gonads. In the sense 

 indicated above there is much to be said in favour of Hatschek's inter- 

 pretation : the other idea is wildly impossible. 



