NOTES ON ELASMOBRANCH DEVELOPMENT. 259 



loose mesh, and its fibres are always delicate ; and it is no 

 doubt often torn and destroyed by the preserving processes to 

 which the embryo has to be subjected. But delicate as it is, 

 there can be no doubt of its existence in Vertebrate embryos ; 

 and there can be no reasonable doubt that it is derived from 

 the processes and strands left between the cells as a result of 

 the incomplete cleavage of the ovum. There can be no doubt, 

 I say, that the network exists ; but that the peripheral nerve- 

 fibres and the central nerve-fibres are derived from it has not 

 yet been shown. That is the point which now needs investi- 

 gation, and I hope myself to treat of it in a future paper. 



Meanwhile I may say that there is in my opinion evidence 

 to show that the whole of the nervous connections (by nerve- 

 fibres and otherwise), both in the central organ and at the 

 periphery, are developments of this pre-existing network, which 

 connects together at all times the whole of the cells derived 

 from the fertilised ovum. 



I do not dispute for one moment the description given by 

 Dohrn^ of the structure of particular stages in the development 

 of a nerve-fibre ; but in saying that it consists of a row of ecto- 

 derm-cells laid on end to end he is, I think, going beyond his 

 facts, being led to such an interpretation of the appearances 

 not so much by observation of previous stages as by a process 

 of reasoning based upon the cell theory of structure, which 

 theory implies that the animal body at one stage of its onto- 

 geny consisted of cells which are separate from one another 

 and only secondarily fuse to form the adult tissues and com- 

 binations. 



' ' Studien z. Urg. d. Wirbelthierkorpers,' No. 17. 



