History of a transient nervous apparatus in certain Ichthyopsida. 361 



the centre. Two such groups and their nerve-processes are depicted 

 in the figure, and in Fig. 98 there is another similar group to be seen. 



5. Embryos of 33— 37,5 ram. 



A few of the general characters of such embryos are as follows. 

 The embryo is still growing in length, and as yet but little in width, 

 for the fins have not yet begun that lateral expansion which in later 

 stages causes Raja embryos to differ so markedly in appearance from 

 most other Elasmobranch embryos; in other words, at present the 

 embryo is still more rounded than flattened. The external gills of 

 the five arches measure from 1,5 to 2 mm in length; in comparison 

 with those of embryos of double the size they are still very small. 

 The two unpaired caudal fins — the only unpaired ones which ever 

 appear in Raja — are developing, and muscle-buds are being given 

 off into them. 



From embryos within the above dimensions sections have been 

 figured from No. 209 (34 mm), No. 219 (37,5 mm) and from hori- 

 zontal series of Nos. 435 and 214 (33 mm). Figs. 36 and 37 (embryo 

 No. 209) on plate 23 may firstly be referred to in connection with 

 the question as to the condition of the centrally-lying cells of the 

 transient system at this stage. These appear to be as large and well- 

 nourished as ever. The protoplasm is granular, the nucleus large, 

 and there are in each nucleus two or more large refractile nucleoli. 

 In short, they exhibit as yet no signs of degeneration. Capsule cells 

 aie of course present as in earlier stages, and, moreover, such cells 

 are now found closely applied to those ganglion-cells of the transient- 

 system which have found a resting-place in the mesoderm. The 

 nerves of the apparatus are numerically as well represented as ever, 

 and, as depicted in Figs. 36 and 37, they, too, now may have nuclei 

 applied to them at intervals in their course. In this connection 

 Fig. 37 may be cited. Here is to be seen a nerve (s.n) whose begin- 

 ing in a ganglion-cell was not ascertained; but it will be noticed 

 that in its course over the tip of the myotome this nerve has nuclei 

 applied to it, A similar nerve on the opposite side has a thick 

 coating of ganglion-cells. In Fig. 36 one section only was drawn and 

 thus only small portions of the two nerves were obtained, but sufficient 

 of it is there to demonstrate the "capsule-cells" {c.c) on both nerve 

 and ganglion-cell. 



The remaining figure from embryo No. 209 is Fig. 74 on plate 25. 

 This figure was carefully drawn under the best lens at my disposal, 



