History of a transient nervous apparatus in certain Ichtbyopsida. 423 



Plate 26. 



Fig, 75. A transverse section from embryo No. 563. To show 

 the origin and course of a pair of transient nerves. The figure is from 

 one section, but to the right a piece of the nerve opposite the myo- 

 tome is filled in from another section. Magnif. 225. 



Fig. 76. A portion of a transverse section from embryo No. 551. 

 To show the spinning of fibres, the projection of processes to the epi- 

 blast , and the crossing of fibres from side to side in the centre. 

 Magnif. 500. 



Fig. 76 a. From the same embryo. A ganglion-cell with nerve- 

 process reaching to the epiblast. Magnif. 500. 



Fig. 77. From a section of the spinal cord near the tail-end of 

 embryo No. 237 (45 mm). To demonstrate the presence of transient 

 ganglion-cells in the caudal region. Magnif. 500. 



Fig. 77 a. A transient ganglion-cell of the caudal region as above. 

 Embr. No. 255 (71 mm). Magnif. 225. 



Fig. 77 b. Three such cells from the roof of the cord near the 

 tail-end of the same embryo. Magnif 500. 



Fig. 78 a — g. A series of figures to illustrate early stages of 

 nerve-spinning by transient ganglion- cells. The figures are from three 

 embryos. Magnif. 500. NB. Figs. 87 and 88 yield slightly older 

 stages of this process and help to complete the series. 



Figs. 79, 80, 81. A series of figures to show the transition from 

 ganglion-cells to "nerve-cells" or nerve-forming cells. All are elements 

 of the transient system. Fig. 79 a — h from embr. No. 156 (12 mm), 

 k and Fig. 81 from embr. 192, and Fig. 80 from R. clavata No. 226 

 (33 mm). Magnif. 500. 



Fig. 82. Portion of a horizontal section of the dorsal region of 

 the cord of embr. No. 435 (33 mm), showing the multipolar nature of 

 the transient ganglion-cells. Magnif. 500. 



Fig. 83. A bi-polar ganglion-cell from embr. No. 214 (33 mm). 

 Magnif. 500. 



Fig. 84. Portion of a section, similar to that figured in Fig. 82. 

 Embr. No. 433 (16 mm). Magnif. 500. 



Fig 85. A section through the apex of a myotome in which four 

 transient cells are embedded. Of these one is unquestionably a ganglion- 

 cell, the others are doubtfully such. Embr. H. radiata No. 236, 33 mm. 

 Magnif. (Lbitz pantachr. 2 mm) 500. 



Fig. 86. A ganglion- cell embedded in the apex of the myotome. 

 No nerve in connection. Embr. No. 201 a, 21 mm. Magnif. 225. 



Fig. 87. A transient nerve in course of formation. From the 

 ganglionic node at the apex of the myotome a fibre {n.p) is being spun 

 along the outer side of the myotome. Embr. No. 564, 10,5 mm. 

 Magnif. 500. 



Fig. 88. A spinning ganglion-cell and its fibre from embr. No. 564, 

 Magnif. 500. 



