[131] The reader is referred for this subject to the valuable memoirs which have been recently published by the Hertwigs, especially to No. 270. He will find a general account of the subject written before the appearance of the Hertwigs' memoir in pp. 180-182 of Volume II. of this treatise. [132] It would be interesting to know the history of the various nervous structures found in the walls of the alimentary tract in the higher forms. I have shewn (_Development of Elasmobranch Fishes_, p. 172) that the central part of the sympathetic system is derived from the epiblast. It would however be well to work over the development of Auerbach's plexus. [FIG. 207. TWO TRANSVERSE SECTIONS THROUGH EMBRYOS OF HYDROPHILUS PICEUS. (After Kowalevsky.) A. Section through an embryo at the point where the two germinal folds most approximate. B. Section through an embryo, in the anterior region where the folds of the amnion have not united. _gg._ germinal groove; _me._ mesoblast; _am._ amnion; _yk._ yolk.] For further light as to the origin of the mesoblast, it is necessary to turn to its actual development. The following summary illustrates the more important modes in which the mesoblast originates. [FIG. 208. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. (From Parker; after Reichenbach.) A. Section through part of the ovum during segmentation. _n._ nuclei; _w.y._ white yolk; _y.p._ yolk pyramids; _c._ central yolk mass. B. and C. Longitudinal sections of the gastrula stage. _a._ _en._ archenteron; _b._ blastopore; _ms._ mesoblast; _ec._ epiblast; hypoblast, distinguished from epiblast by shading. D. Highly magnified view of anterior lip of blastopore, to shew the origin of the primary mesoblast from the wall of the archenteron. _p.ms._ primary mesoblast; _ec._ epiblast; _en._ hypoblast. E. Two hypoblast cells to shew the amoeba-like absorption of yolk spheres. _y._ yolk; _n._ nucleus; _p._ pseudopodial process. F. Hypoblast cells giving rise endogenously to the secondary mesoblast (_s.ms._); _n._ nucleus.] 1. It grows inwards from the lips of the blastopore as a pair of bands. In these cases it may originate (_a_) from cells which are clearly hypoblastic, (_b_) from cells which are clearly epiblastic, (_c_) from cells which cannot be regarded as belonging to either layer. Mollusca.--Gasteropoda, Cephalopoda, and Lamellibranchiata. In Gasteropoda and Lamellibranchiata the mesoblast sometimes originates from a pair of cells at the lips of the blastopore, though very probably some of the elements subsequently come from the epiblast; and in Cephalopoda it begins as a ring of cells round the edge of the blastoderm. Polyzoa Entoprocta.--It originates from a pair of cells at the lips of the blastopore. Chætopoda.--Euaxes. It arises as a ridge of cells at the lips of the blastopore (fig. 200).