History of a transient nervous apparatus in certain Ichthyopsida. 367 



Only one figure (Fig. 40, plate 23) has been drawn from embryo 

 .No, 237. It shows some of the central cells {gl. c. c) with their capsule- 

 cells, as also a group of "mesodermal" ganglion-cells {w.gl.c) with 

 capsule-cells. 



Speaking generally of the transient system of this embryo, its 

 nerves are as numerous as in No. 229, and have the same course 

 and characters: similar ganglionic elements are met with here and 

 there in the myotomes, and odd ganglion-cells, as well as groups of 

 such, can be noted in the mesoderm. This series and the preceding 

 one (No. 229) also exhibit scattered here and there in the mesoderm 

 small rounded cells which may possibly be cells belonging to the 

 transient system, cells which have not taken on ganglionic characters 

 at any period of the development, and which are now in process of 

 degeneration. It is, however, difficult to decide as to their real nature, 

 and in this part of the work, which is devoted solely to a record of 

 observation, the statement merely of their occurrence is in place, 

 while the enunciation of hypothetical views with reference to them 

 may be deferred. Before passing on to the description of older 

 embryos another matter may be here briefly mentioned. 



All along we have had under review only that part of the ap- 

 paratus, which is first developed, and which was stated to extend 

 over some 24 — 25 trunk somites. For some time those in this region 

 are the only transient elements present, but by and bye one also 

 meets with scattered ganglion-cells on the cord further caudalwards 

 and in the tail itself^). Such is the case in embryos of 20 mm and 

 upwards. Now, Raja embryos are furnished with a long tail, which 

 is always getting bigger and bigger the older the embryo becomes, 

 until in an embryo of 43 mm it occupies considerably more than half 

 the total length of the animal, and again, in embryos of 14 cm, for 

 instance, the tail from the end of the pelvic fins to its termination 

 measures 9 cm. Long and laborious as the task would have been, it 

 might have been necessary to have made minute investigations into the 

 development of the transient nervous apparatus in this part of the 

 body also, had not the fact become manifest that here the system 

 never attains a very high degree of development. One may, therefore, 

 be content to record the occurrence of transient ganglion- cells in the 

 spinal cord posterior to that portion already dealt with and along 

 the tail. The degeneration of these elements could also be established, 



1) A fuller discussion of this matter will be found in the "general 

 summary of the development of the apparatus etc." 



