History of a transient nervous apparatus in certain Ichthyopsida. 339 



In Fig. 16 some ganglion-cells are found migrating from the cord. 

 Of these two {w. gl. c) are linked together and appear to be forming 

 a nerve by what, in contradistinction to the "process" mode, may be 

 described as the "cellular" form of nerve formation. 



Fig. 17 bears a family resemblance to Fig, 12 (No. 141), in that 

 on both sides there are cells just beginning the spinning of fibres. 

 On the left side the ganglion- cells are somewhat linked together. 



Fig. 15 depicts a bipolar ganglion - cell (gl.c) of the transient 

 series. In both directions, i. e. towards the centre and towards the 

 periphery it has spun out its nerve-process. The appearance of the 

 nucleus, i. e. absence of highly refractile nucleoli, suggests that this 

 cell would sink to the significance of a mere nerve forming cell, such 

 as that shown in Fig. 44. 



Here and there are odd spun-fibres, and a few instances of 

 bunches of ganglion-cells reaching to the tip of the myotome. The 

 central ganglion-cells at any rate are only in the act of becoming 

 ganglionic. 



What has been said of this series largely holds of another (No. 155) 

 in which 90 somites had arisen. 



No necessity appears to exist for its detailed description, but 

 attention may be directed to one figure from it. This (Fig 1) may 

 be cited as demonstrating phases of the development of ganglion-cells 

 to such condition. The cells of the future spinal ganglia in the pro- 

 nephric region are still only in process of migration from the lips of 

 the cord. As in Fig. 5, among them are some few recognisable al- 

 ready as belonging to the transient system. This is shown by in- 

 crease of size of the nucleus and the presence of large refractile 

 nucleoli, and by other characters. Three other cells, to the left of the 

 figure, may be looked upon as already ganglionic, and two of them 

 have spun out nerve-processes. 



Passing now to embryos of 10 — lOi mm. Nos. 163 and 164 may 

 first be mentioned. 



In Nos. 163, 164 the number of somites counted was 100—101. 

 The lens is a thickening of the skin. The last gill-pouch is just form- 

 ing, and three clefts are open in No. 163, whilst the spiracular pouch 

 has only ruptured on one side in No. 164; there are very slight ex- 

 ternal gill-buds on the second and third arches. Neither series shows 

 anything approaching the remarkable appearances of No. 141, nor of 

 No. 410. 



Zool. Jahrb. IX. Abth. f. Morph. 23 



