History of a transient nervous apparatus in certain Ichthyopsida. 329 



and ganglion- cell, as also between two or more of the latter are found 

 to occur, but do not appear to be invariable. 



These macro- or transient nerves, and other nerves also, are 

 merely transformations of ganglion-cells. All stages in this metamor-- 

 phosis can be noted. The capsule cells arise from epiblast cells of 

 the cord laying just beneath and around the macro-ganglion cells. 

 They, too, are to be looked upon as modified ganglion-cells, which 

 have lost their specifically ganglionic functions. 



Soon after the 45 mm stage is reached, it becomes difficult, owing 

 to the great increase in the formative tissue of the mesoblast (Bil- 

 dungsgewebe of Götte and Ziegler), to follow the transient nerves. 



The involution of the transient nerves would appear to commence 

 at the epoch named in my Lepidosteus paper, i. e. with the formation 

 of the permanent central canal of the cord. On this point it is ne- 

 cessary to correct an error in the paper just mentioned. The processes 

 of the ganglion-cells there spoken of have turned out to be either 

 nerves (axis-cylinders) or the remains of such. These are not 

 "cut off", but, like the macroganglion-cells themselves, 

 they undergo a gradual simple atrophy, and traces of 

 them can be seen for a long time, especially in the 

 form of shrunken processes from poles of theganglion- 

 cells. The latter persist for a much longer period, and, indeed, 

 have not entirely disappeared in newly hatched R. hatis of some 

 20 centimetres in length. The lengthened period, which complete 

 atrophy would appear to require in this case, may be judged of, 

 when it is stated that the process began during the 5th month of 

 life within the egg, while traces of the cells could still be found in 

 skate, which hatched out some 17 months after the fresh egg was 

 taken from the parent. 



There is sometimes present, but as yet it has not be shown to 

 exist in every embryo, a subepithelial (subepiblastic) system of nerves, 

 possibly of a sensory nature. It arises in connection with ganglion- 

 cells of the cord (as an outgrowth of cells of the latter) slightly 

 ventrad of the motor macro-ganglion-cells. This system has also been 

 seen in embryos of 45 mm in length, but its complete history can be 

 at present only a matter of surmise. Further investigation is neces- 

 sary ; there can, however, be no doubt of its larval and transient 

 nature." 



For the sake of completeness a few other recent papers, in which 

 the ganglion-cells in question are incidentally, accidentally, or other- 



