Fig. 275. Transverse section through the anterior part of the trunk of an embryo of Scyllium slightly older than fig. 29 B.
The section is diagrammatic in the fact that the anterior nerve-roots have been inserted for their whole length; whereas they join the spinal cord halfway between two posterior roots.
sp.c. spinal cord; sp.g. ganglion of posterior root; ar. anterior root; d.n. dorsally directed nerve springing from posterior root; mp. muscle plate; mp´. part of muscle plate already converted into muscles; mp.l. part of muscle plate which gives rise to the muscles of the limbs; nl. nervus lateralis; ao. aorta; ch. notochord; sy.g. sympathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric) duct; st. segmental tube; du. duodenum; pan. pancreas; hp.d. point of junction of hepatic duct with duodenum; umc. umbilical canal.
Bibliography of the Sympathetic Nervous System.
(360) F. M. Balfour. Monograph on the development of Elasmobranch Fishes. London, 1878, p. 173.
(361) S. L. Schenk and W. R. Birdsell. “Ueb. d. Lehre von d. Entwicklung d. Ganglien d. Sympatheticus.” Mittheil. a. d. embryologischen Instit. Wien. Heft III. 1879.
[152] Whether there is any part of it in many types not so derived requires further investigation, now that it has been shewn by the Hertwigs that part of the system develops from the endoderm in some Cœlenterata. O. Hertwig holds that part of it has a mesoblastic origin in Sagitta, but his observations on this point appear to me very inconclusive. It would be very advantageous to investigate the origin of Auerbach’s plexus in Mammalia.
[153] Our knowledge on this subject is especially due to the brothers Hertwig (Nos. [320] and [321]), Eimer (No. [318]), Claus (No. [317]), Schäfer (No. [326]), and Hubrecht (No. [323]).
[154] Reichenbach (No. [331]) holds that the walls of the groove between the two strands of the ventral cords become invaginated and assist in the formation of the ventral cord.
[155] “Ueber Entwicklungsgeschichte d. Echiurus.” Arbeit. a. d. zool. Instit. Wien, Vol. III. 1880.
[156] Vide Vol. II., pp. 273, 274.
[157] “Ueber Entwicklungsgeschichte von Teredo.” Arbeit. a. d. zool. Instit. Wien, Vol. III. 1880.
[158] For the development of the central nervous system in Amphioxus and the Tunicata the reader is referred to the chapters dealing with those two groups.