History of a transient nervous apparatus in certain Ichthyopsida. 383 



the fact of an early formation of nerve-processes spun by the ganglion- 

 cells themselves. This is especially true of the centrally-lying ganglion - 

 cells. As seen in both horizontal and transverse sections of embryos 

 of 6 — 10 mm, whilst the cells at the top of the cord are taking on 

 ganglionic characters, they are also engaged in spinning out processes 

 (Figs. 1 and 3). In some embryos very early stages of this operation 

 may be discovered. In this connection embryos Nos. 81, 141 and 143 

 may be specially cited, and a reference to Figs. 9, 10, 11, 12, 17, 

 20 and Fig. 78 a— g may be made. 



His is the only author who has figured examples of "nerve- 

 spinning" by ganglion-cells, and as the number of his figures is very 

 limited, it may be advisable to point out and illustrate a few of the 

 more salient features of the process. 



The protoplasm of the cell is projected forwards, as if the cell 

 were about to migrate. At first this outgrowth of the cell is thick, 

 more or less blunt, and striated (Fig. 78 a— g, plate 26), but, as it 

 grows in length, it thins out and becomes much finer, the striation 

 persisting to a certain extent (Figs. 10 and 13). 



When it commences to be formed, the nerve-process is often as 

 thick as the parent-cell, but this is soon altered, and then the cell- 

 body tapers in a pear-shaped fashion in the passage into its process 

 (Fig. 78 g. Fig. 87 and Fig. 88). 



The processes thus spun by the central ganghon-cells of the 

 system all tend either towards the epiblast, or in the direction of the 

 apex of the myotome (Figs. 12, 15, 20, 87 etc.), and many of them 

 finally make their way along its outer side (Fig. 75). In their course 

 outside the myotome these nerves lie very closely under the epiblast, 

 and many instances of short nerve - processes have been observed, 

 passing like those in Figs. 25, 55 and 76 direct to the epiblast just 

 above the cord. 



Although these sub-epiblastic nerve-processes often put in an 

 appearance at a very early stage, this is not the case with every 

 embryo examined. For, as previously mentioned (page 351), older 

 embryos frequently exhibit only the earliest stages of the development 

 of the apparatus and of its nerves, indeed, it seems probable that in 

 some embryos only does the system of sub-epiblastic nerves attain a 

 high degree of development. But , during the stages which precede 

 the degeneration, there are always indications, at any rate in embryos 

 of 10 mm and upwards, of the existence of such nerves, even though 

 they be merely represented by but short spun processes of the ganglion- 



