274 
W. M. Wheeler 
Ovum, it is quite possible that it may be altogether absent. In these 
mstances the appearance of the cell-group is strikingly like that of 
the oocyte with its Nährzelle in Ophryotrocha. It may be noted, 
moreover, that there is really a bipolar arrangement of the Nähr- 
zellen in Diopatra and that it is only necessary to imagine each of 
the cell-strings of that Terebellid reduced to a single cell to have 
the condition seen in Myzostoma. The difference between Myzostoma 
and Diopatra would be no greater than that which obtains between 
the two Gephyreans Bonellia and Thalassema, In the second place 
I can See no reason for abandoning the homology on the ground that 
the accessory cells are not cast off. There is this difference between 
Polychaeta and Myzostoma that in the latter the growing eggs are 
stowed away in compact masses in the diverticula of the body-cavity, 
whereas in the former they can usually move about more freely. 
Hence there is really no opportunity for the accessory cells to leave 
the ova, and it would seem quite natural that they should be ap- 
propriated as food^. 
Nährzellen are known to be of very general occurrence in the 
Arthropoda. They have long been known to occur in Crustacea, 
especially in Sacculina^ where they have been repeatedly described 
(Gerbe '69; Ed. van Beneden '69, 70a, 70b; Balbiani '69; Perez '78; 
Delage '84); v. Siebold has observed them in Apus ('71), Weis- 
mann in Daphnia ('85) and Brauer in Branchipus ('92). In Insects 
they have been very carefully studied by Korschelt ('86) to whom 
the reader may be referred for a full account of the subject. The 
trophic function of the Näh rz eilen can hardly be doubted in 
Arthropods where these cells are consumed by the oocytes, but it is 
more difficult to understand in Annelids, where the egg presents a 
far greater surface to the nutrient fluids of the body-cavity than to 
the Nährzelle. In many Polychaeta, too, the Nährzellen appear to 
be absent, although the growth of the ovura seems to be completed 
quite as easily as it is when they are present. These considerations 
show that a discussion of the views of the different authors, concern- 
1 I must confess that although I have taken great pains to ascertain the 
fate of the accessory cells, there is still a possibility that they are not all 
appropriated by the oocytes. One often finds small, flattened and deeply stain- 
ing cells which form follicle-like partitions between the ova. Whether these 
arise from some of the accessory cells or from the parenchyma and peritoneum, 
I am unable to say. 
