340 INSECT A. 



rectly from aquatic ancestors not provided with tracheae. But although this 

 suggestion of Fritz Miiller cannot be accepted, it is still possible that the 

 processes discovered by him may have been the earliest rudiments of wings, 

 which were employed first as organs of propulsion by a water-inhabiting 

 Insect ancestor which had not yet acquired the power of flying. 



The nervous system. The nervous system arises entirely from 

 the epiblast ; but the development of the prse-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. 1 89 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 inandibular 

 segment the median element is absorbed, and that the two lateral cords 

 of that part give rise to the cesophageal commissures, while the sub- 

 cesophageal ganglion is formed from the fusion of the ganglia of the two 

 maxillary segments. * 



The prae-oral portion of the nervous system consists entirely of the 

 supra-cesophageal 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 in- 

 vaginated on each side close to the dorsal border of the antenna?. 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-cesophageal ganglia remain disconnected on the dorsal 

 side till quite the close of embryonic life. 



The tracheae and salivary glands. The trachea?, as was first 

 shewn by Biitschli (No. 405), arise as independent segmentally arranged 

 paired invaginations of the epiblast (fig. 189 B and 0, st). Their 

 openings are always placed on the outer sides of the appendages of 

 their segments, where such are present. 



