88 OLIGOCHAETA 



in an ovary; they have been bodily transferred to the egg-sac. In Libyodrilus this 

 appears to be the case ; I succeeded in finding the ovaries in a very young worm, 

 but not in mature or even in nearly mature specimens ; no one has detected the 

 ovary of Polytoreutus, though its position has been probably fixed with accuracy; 

 in Stuhlmannia, too, the mass of cells described by MICHAEI.SEN as an ovary may 

 be one, but it has none of the characteristic appearances of germinal tissue ; I am 

 of opinion that this is to be explained on the assumption that all, or nearly all, of 

 the germinal cells have been made over to the egg-sacs. 



The facts evidently require another explanation. It might be thought that there 

 were differences in the development of the embryos sufficient to account for this. 

 If, as one might perhaps infer from D'UDEKEM'S figures, the eggs when large and 

 full of yolk completely filled the cocoon to the exclusion of any albumen, it would 

 be at once apparent that the absence of a nutritive fluid necessitated other nutriment 

 for the developing egg ; but VEJDOVSKY has specially described the albumen in the 

 cocoon of Rhynchelmis an Oligochaet with very large ova. A free larval stage in 

 one group or the other might also get over the difficulty ; but there is none such ; 

 in all the Oligochaeta whose development is known not a very large series it must 

 be admitted, but still a series comprising representatives of genera with large and 

 genera with small ova, the young leave the cocoon at approximately the same age ; 

 there are at least no striking differences in this particular. 



Among the vertebrata it is always possible to trace some connexion between 

 the abundance of yolk in the ovum and the needs of the embryo ; for example, the 

 ova of Amphioxus have little yolk and are minute in size ; this is correlated with 

 the fact that the young are hatched in an immature condition ; in the mammals 

 there are of course special provisions (the placenta) to prevent the otherwise early 

 hatching of the embryo ; the frog which has much more yolk leaves the egg in a 

 tolerably advanced stage of development ; finally the bird and reptile is hatched in 

 the practically adult condition. Nothing of the kind occurs in the Oligochaeta. 



I formerly attempted to show that there was truth in the idea that the nature 

 of the ova is correlated with the habitat of the worm ; the ova of Allurus which is 

 an aquatic form though related to the terrestrial genera are apparently larger than 

 those of Lumbricus ; the difference, however, is not by any means an obvious one ; 

 moreover, I since found that the nearest approach to the ova of the ' Lirnicolae ' 

 among the Terricolae was to be seen in Moniligaster ; here we have ova which 

 are filled with large yolk spherules as large as those of, for instance, Tubifex ; 

 Moniligaster is purely a terrestrial genus so far as we know. 



The mature egg of Rhynctidmis, which may be selected as the type of a large 



