History of a transient nervous apparatus in certain Ichthyopsida. 323 



on in a gradually ascending path, without any turnings, temporary 

 expedients, or retrogressive changes. As a consequence none were 

 looked for and none found. Every observer is under obligations to 

 his predecessors, often even when their results only relate to fields 

 widely separated from his own little plot of ground. And in the 

 present case. The stimulus needed in order that this a priori un- 

 productive research should be pursued to a fitting termination was 

 found in Kleinenberg's epoch-making researches on Annelidan deve- 

 lopment ^). It was his memoir which first gave a prominence, not 

 warranted by their own modest appearance, to the cells outside the 

 developing nervous system, of which the present writing treats. It 

 was his example that furnished the patience necessary for a six years 

 investigation into the characters and history of these apparently in- 

 significant cells. 



The, as we now know them to be, transient ganglion-cells were 

 first stumbled across by the present writer in Lepidosteus, and the 

 fruits of a preliminary study of their characters and history in this 

 and several other Ichthyopsida were published in a small paper in the 

 Proceedings of the Royal Society, London 1889 ^). 



In the same year it was ray good fortune to find in Baja hatis 

 a valuable object for their further investigation. Since then material 

 has accumulated, and a range of stages of the whole development of 

 this Elasmobranch has been obtained such as rarely falls to the lot 

 of the embryologist. Other duties and work in other fields have 

 hitherto prevented the final publication of the results gleaned. Not, 

 however, without a corresponding advantage; for, at any rate, the 

 delay must have left fewer errors of observation to be corrected by 

 others, and the results have matured under constant observation and 

 reflection. 



In the course of six years the facts have been revised again and 

 again, and the cessation of new discovery in the research appears to 

 prove that, for myself at least, there is in Raja little more to be 

 found, and that thus publication is called for. 



In the following pages a strictly objective mode of description 

 will be employed, and as far as possible theoretical considerations will 

 be kept out of account. Many of the facts to be adduced are for 



1) No. 7 in the list of literature. 



2) No. 2. 



Zool. Jahrb. IX. Abth. f. Morph. 22 



