History of a transient nervous apparatus in certain Ichtliyopsida. 391 



As the process follows along exactly the same lines of degene- 

 ration as pursued by the central ganglion-cells, no detailed account of 

 it seems called for: there are, however, one on two special examples 

 worthy of note. In Fig. 27 a group of such cells from the mesoderm 

 in an advanced stage of degeneration is sketched. As stated in the 

 description of embryo No. 255, they were found near the epiblast on 

 the level of the notochord. In the section they have a glassy ap- 

 pearance, their outlines are sharply defined and irregular. In one of 

 them there is a marked tendency to the formation of fissures in the 

 protoplasm. Their nuclei are exceedingly faint, and possess hardly 

 any contents. Comparison of the picture of these cells with that 

 (Fig. 104) of a group of central cells from a slightly larger embryo 

 brings home to one the fact of the degeneration more vividly than 

 any description. 



The group of Fig. 27 belongs to the category of those erratic items 

 of the transient apparatus which, while still putting in an occasional 

 appearance in the development, can hardly be considered as still of 

 functional value to the apparatus. They are probably vestigeal in 

 character. 



Among others of the peripheral cells of similar nature the very 

 early degenerating cells of Figs. 59 and 67 may be also cited. In 

 the embryo (No. 229) in which they are present, the central ganglion- 

 cells exhibit no signs of degeneration. They lie in the myotome, and 

 it has already been suggested that ganglion-cells in this position may 

 possibly be vestigeal and of no present functional value as integral 

 parts of the apparatus. The same may also be said of the curious 

 group of cells in Fig. 60. The last instance to be briefly noticed is 

 that depicted in Fig. 125. This is a very interesting case of the 

 degeneration of a cell just outside of the myotome. The cell {w.gl.c) 

 depicted had become surrounded by a connective tissue capsule before 

 degeneration set in, and now, in the shrunk and withered condition 

 of the cell, the size of the capsule, which it no longer half fills, tells 

 the story of the former dimensions of the ganglion-cell. 



c) The central cells. To follow out the degeneration of these 

 it is necessary to have sections of embryos of large size — embryos 

 of from 6 to 19 cm. 



Of course it could not be expected that the investigator would section 

 such embryos from end to end. The formation of complete series of 

 eariier embryos entailed quite sufficient labour without the imposition 

 of this additional burden. The procedure adopted was the following. 



