The nervous system. The nervous system arises entirely from the epiblast; but the development of the præ-oral and post-oral sections may be best considered separately.
The post-oral section, or ventral cord of the adult, arises as two longitudinal thickenings of the epiblast, one on each side of the median line ([fig. 189] B, vn), which are subsequently split off from the superficial skin and give rise to the two lateral strands of the ventral cord. At a later period they undergo a differentiation into ganglia and connecting cords.
Between these two embryonic nerve cords there is at first a shallow furrow, which soon becomes a deep groove ([fig. 189] C). At this stage the differentiation of the lateral elements into ganglia and commissures takes place, and, according to Hatschek (No. [414]), the median groove becomes in the region of the ganglia converted into a canal, the walls of which soon fuse with those of the ganglionic enlargements of the lateral cords, and connect them across the middle line. Between the ganglia on the other hand the median groove undergoes atrophy, becoming first a solid cord interposed between the lateral strands of the nervous system, and finally disappearing without giving rise to any part of the nervous system. It is probable that Hatschek is entirely mistaken about the entrance of a median element into the ventral cord, and that the appearances he has described are due to shrinkage. In Spiders the absence of a median element can be shewn with great certainty, and, as already stated, this element is not present in Peripatus. Hatschek states that in the mandibular segment the median element is absorbed, and that the two lateral cords of that part give rise to the œsophageal commissures, while the sub-œsophageal ganglion is formed from the fusion of the ganglia of the two maxillary segments.
Fig. 189. Three transverse sections through the embryo of Hydrophilus. (After Kowalevsky.)
A. Transverse section through the larva represented in fig. 187 A.
B. Transverse section through a somewhat older embryo in the region of one of the stigmata.
C. Transverse section through the larva represented in fig. 187 B.
vn. ventral nerve cord; am. amnion and serous membrane; me. mesoblast; me.s. somatic mesoblast; hy. hypoblast(?); yk. yolk cells (true hypoblast); st. stigma of trachea.
The præ-oral portion of the nervous system consists entirely of the supra-œsophageal ganglion. It is formed, according to Hatschek, of three parts. Firstly and mainly, of a layer separated from the thickened inner part of the cephalic lobe on each side; secondly, of an anterior continuation of the lateral cords; and thirdly, of a pit of skin invaginated on each side close to the dorsal border of the antennæ. This pit is at first provided with a lumen, which is subsequently obliterated; while the walls of the pit become converted into true ganglion cells. The two supra-œsophageal ganglia remain disconnected on the dorsal side till quite the close of embryonic life.
The tracheæ and salivary glands. The tracheæ, as was first shewn by Bütschli (No. [405]), arise as independent segmentally arranged paired invaginations of the epiblast ([fig. 189] B and C, st). Their openings are always placed on the outer sides of the appendages of their segments, where such are present.