History of a transient nervous apparatus in certain Ichthyopsida. 389 



little change, except in a slightly increased complication of its nerves, 

 for a considerable time. 



Often well-developed in embryos of 9 mm in length, in which 

 other organs of the animal are only in their very beginnings, it persists 

 intact until the embryo is 43 mm or more in length, and about then 

 reaches its culmination. 



The period of functional activity of this apparatus is in Raja, 

 as judged by its anatomical characters, a very prolonged one, far 

 longer than in any other form known to me. In many fishes, like 

 Lepidosteus and some Teleosteans, the months of its life-period in 

 the skate are represented by weeks. Under the conditions in which 

 the embryos of Raja were reaied, i. e. in the water of a harbour or 

 in tanks, the system remains quite normal for two months or there- 

 abouts. Then in Raja it enters upon the long slow path of degene- 

 ration, the end of which is reached in an additional twelve months 

 or more. 



Very slight traces of it do appear to persist in the adult form, 

 as will be demonstrated on another occasion , but , as a whole , the 

 apparatus is of an evanescent character, and the whole ^) of what had 

 developed in embryos of 45 mm degenerates and disappears. 



Raja hatis is not a specially suitable form in which to study the 

 process of degeneration. Its remarkable slowness in the skate places 

 difficulties in the way of obtaining all the requisite stages. Again, 

 owing to the central ganghon-cells becoming more or less wedged 

 into the posterior fissure of the cord, two other factors, unfavourable 

 to the observer, come into play. Of these the more obvious is the 

 alteration in shape, which the ganglion-cells suifer by the pressure to 

 which they are subjected ; while the other and less apparent one is 

 the obscurity in which the withering nerves-processes of the cells 

 become enshrouded. 



These facts by way of apology for the circumstance that the 

 details of the degeneration are somewhat cursorily treated of here. 

 Perhaps, however, excuse is superfluous, for the embryologist as such 

 is only concerned in establishing and demonstrating the fact of the 

 complete degeneration of the apparatus. But it cannot be overlooked 

 that the facts of a normal degeneration of ganglion- cells may be of 

 interest to the histologist, and even to the pathologist. The latter 

 probably already knows all that can be gathered about the atrophy 



1) Compare page 398 — 401. 



26* 



