392 JOHN BEARD, 



Apart from many portions of embryonic tails made use of, small pieces 

 taken from the region overlapi)iug and just posterior to the gills and thymus 

 of the various embryos were sectioned. Experience had shown that here 

 only the presence of the ganglion-cells could be with certainty relied upon. 

 The stages of the degeneration are depicted in figures on plates 26, 

 28 and 29, more especially on the two latter. 



Figs. 77 and 77 a of plate 26 and most of those on plate 28 are 

 intended to show the topography, as well as the structure, of the 

 ganglion-cells during the stages of degeneration, while on plate 29 a 

 series of figures is given depicting mainly single cells under high 

 magnification. 



In Figs. 102, 104 and 114 the cells are represented as they 

 appear shortly before atrophy commences. Degenerative changes are 

 first noticeable in the central ganglion-cells of embryos of 7 cm or 

 thereabouts. The nuclear contents become somewhat indistinct, the 

 cells glassy, and somewhat deformed in shape. A sudden withering 

 of their nerve- and other processes is also observable. From this point 

 onwards, as long as they still persist, the cells show an ever-increasing 

 tendency to lose all shape and form. It is a curious, but perhaps 

 not inexplicable, circumstance that the commencement of the atrophy 

 only slightly precedes the changes which lead to the formation of the 

 permanent central canal. In a former paper on Lepidosteus the two 

 phenomena were put into somewhat close relationship, but what justi- 

 fication there may exist for this is rather a question for discussion 

 at a later stage after the history of the transient cells in other forms 

 has been recorded. 



Raja hatis dififers markedly from some of these latter in this 

 respect, that the transient ganglion- cells of this form exhibit a fatal 

 proneness to becoming wedged into the posterior fissure, as demon- 

 strated by Fig. 113 and by others. Owing to the pressure to which 

 the degenerating cells are thereby subjected, their loss of rotundity 

 becomes much exaggerated, their length is increased at the cost of 

 their dimensions in other directions, and, if shape can be ascribed to 

 them, they might be termed spindle-sliaped. The nucleus suflers 

 under the compression, and is seen to be oval and narrow. Their 

 withered processes also become obscured by reason of this external 

 force acting upon them. In short, the general efiect of this disturbing 

 influence is to render it difficult to form a proper conception of all 

 that is taking place in the internal economy of the degenerating cell. 

 Before the cells become much inclosed id the posterior fissure, 



