History of a transient nervous apparatus in certain Ichthyopsida. 359 



as shown in the figure (Fig. 47). The extension of the transient system 

 at this stage will be recorded presently. Fig. 46, pi. 24 offers another 

 instance of those bipolar ganglion-cells of the apparatus which, lying in 

 the mesoderm, are connected with the centre by means of one of 

 their nerve-processes, whilst the other process is directed over the 

 tip of the myotome. 



On the other hand Fig. 47, pi. 24 may be taken as typical of many 

 sections in various embryos. Three centrally lying ganglion-cells {gl. c. c) 

 are depicted in the figure, from one of these a long fine axis-cylinder 

 {n.p) can be seen and this, when followed, is observed to pass over 

 the tip of the myotome, there to become a sub-epiblastic nerve of 

 greater or less extension. 



Another figure from the series (Fig. 86, plate 26) demonstrates 

 something with which the reader is already familiar — a ganglion- 

 cell {gl.c^) embedded in the myotome. In this instance there appeared 

 to be no nerve in connection; that is, the cell neither gave off' pro- 

 cesses, nor was it in contact with the axis-cylinder of another cell. 



From a small portion of a section of the horizontal series No. 202 

 the Fig. 73 of plate 25 is taken. The section passes through the 

 summit of the spinal cord, and, as the myotomes of one side are 

 drawn in, it will be recognised that the figure depicts ganglion-cells 

 for a range of four myotomes. From this series of horizontal sections 

 it was calculated that in the region of each somite at least twenty 

 ganglion-cells of the transient system were met with on the cord. This 

 is, of course, in the region of the body where the system is concen- 

 trated in Baja embryos. The extension of the apparatus at this 

 period, as determined in the horizontal sections of embryo No. 203, 

 is as follows. The ganglion cells begin to put in an appearance 

 opposite the sixth somite from the auditory capsule. They are very 

 numerous opposite the W\ 12 tb, 13 »h, 14 th, 15 th and 16 th somites, 

 and remain so until about the 26 th somite. Then they become fewer, 

 and end somewhere about the region of the 31st segment of the trunk. 



In embryo No. 156, 12 mm, the extension was not so easily 

 determined, but it appeared to be over the region of about 25 somites. 



In the words of the preliminary paper, referring to embryo 

 No. 203, "anteriorly, from segments 6 to 10, these cells are few in 

 number, then, from segments 11 to 26, they come in crowds of 3, 4, 

 or more to each side of the body in nearly every transverse section, 

 intervals of crowding alternating with others in which they are more 

 sparsely collected. Finally, they diminish from the 26th segment 



