EXPLAXA.TIOX OF PLATE XIII. 

 This Plate illustrates the Formation op the Spinal Nerves. 



Comj^lete list of reference letters. 



p r. posterior root of a spinal nerve. 

 a r. anterior root of a spinal nerve. n. spinal nerve. 



spg. ganglion on posterior root of spinal nerve. 

 com. commissure connecting the posterior roots of the spinal nerves. 

 w. white matter of spinal cord. n.c. neural canal. 



y. point where the spinal cord became segmented off from the super- 

 jacent ei)iblast. 



/. mesoblastic investment of spinal cord. m.p. muscle-plate. 

 V. r. vertebral rudiment. ch. notochord. 



Fig. 1, 2, and 3. Three sections of a Pristiurus embryo belonging to 

 stage I. Fig. 1 passes through the heart, fig. 2 througli the anterior part 

 of the dorsal region, fig. 3 through a point slightly behind this. (Zeiss CC, 

 ocul. 2.) In fig. 3 there is visible a slight proliferation of cells from the 

 dorsal summit of the neural canal. In fig. 2 this proliferation definitely con- 

 stitutes two chib-shaped masses of cells {p r) — the rudiments of the posterior 

 nerve-roots, — both attached to the dorsal summit of the spinal cord. In 

 fig. 1 the rudiments of the posterior roots are of considerable length. 



Fig. 4. Section through the dorsal region of a Torpedo embryo slightly 

 older than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The section 

 shews the formation of a pair of dorsal ner\re -rudiments {p r) and a ventral 

 nerve- rudiment {a r). The latter is shewn in its youngest condition, and is 

 not distinctly cellular. 



Fig. 5. Section through the dorsal region of a Torpedo embryo slightly 

 younger than stage K. (Zeiss CC, ocul. 2.) The connective-tissue cells are 

 omitted. Tiie rudiment of the ganglion {sp. g.) on the posterior root has 

 appeared, and the junction of posterior root with the cord is diflScult to 

 detect. The anterior root forms an elongated cellular structure. 



Fig. 6. Section through the dorsal region of a Pristiurus embryo of 

 stage K. (Zeiss CC, ocul. 2.) The section especially illustrates the attach- 

 ment of the posterior root to the spinal cord. 



Fig. 7. Section through the same embryo as fig. 6. (Zeiss CC, ocul. 1.) 

 The section contains an anterior root, which takes its origin at a point 

 opposite the interval between two posterior roots. 



Fig. 8. A series of posterior roots with their central ends united by a 

 dorsal commissure, from a longitudinal and vertical section of a Scyllium 

 embryo belonging to a stage intermediate between L and M. The embryo 

 was hardened in a mixture of osmic and chromic acids. 



Fig. 9. The central end of a posterior nerve-root from tlie same embryo, 

 with the commissure springing out from it on either side. 



