288 EMBRYOLOGY 



In opposition to this theory, Kleinenberg — with whom Goette, 

 Salensky, Bergh, and Fraipont agree — espouses the view that the nerv- 

 ous system is composed of two separate fundaments. The supra-oeso- 

 phageal ganglion arises as a preoral ectodermal thickening independ- 

 ently of the two longitudinal thickenings of the ventral side, which 

 represent the fundament of the ventral nerve cord. (A longitudinal 

 furrow corresponding to the medullary tube of the vertebrates does 

 not exist.) Later it puts forth lateral processes, the oesophageal con- 

 nectives, which unite with the already-formed ventral nerve cord. 

 Such is the condition in Luvibricus. The origin of the nervous system 

 in Lopadorhynchus, likewise studied by Kleinenberg, depends upon 

 much more complicated formative processes. Lopadorhynchus develops 

 from a monotrochal larva, the posterior portion of which grows out into 

 the worm in the manner already described. A ciliated pit, the so- 

 called apical organ (Fig. 135, p. 293), and the two apical tentacles 

 arise in the vicinity of the apical pole as provisional sense-organs. 

 Behind these the two pairs of permanent antennae and the olfactory pits 

 are formed, also as ectodermal growths. From these organs, which later 

 degenerate in part, the formation of the supra-oesophageal ganglion 

 proceeds. Ordinary ectodermal cells are metamorphosed into ganglionic 

 cells, which accumulate in the region of these organs, later sink in 

 deeper, and unite to form the supra-oesophageal ganglion. This finally 

 detaches itself from connection with the ectoderm and appears inside 

 the body as an independent organ. 



Just as the formation of the supra-oesophageal ganglion, according to 

 Kleinenberg, starts from the sensory organs, so the origin of the ventral 

 nerve cord is also referred by this investigator in part to the influence of 

 the sensory organs. In the main, however, the impetus to its formation 

 proceeds from the locomotor organs. From the inner [deep] side of the 

 ventral ectoderm, the outer surface of which bears tufts of sensory hairs 

 (Fig. 134 C), a plate is separated off, the so-called neural plate, in 

 which a right and left portion can be distinguished (Fig. 134, p. 292). 

 Along this plate arise segmental, paired ingrowths, the setigerous sacs 

 (Fig. 134 C). Dorsad and ventrad from these are formed as ectodermal 

 ingrowths the dorsal and ventral cirri. The parts of the neural plate 

 situated nearest to the median plane supply the ventral cord. They 

 approach more and more the middle line, and here fuse with each other. 



The union of the ventral nerve cord with the supra-oesophageal gan- 

 glion is secondary. It is brought about by the neural plates extending 

 forward and sending out processes to the ring- nerve of the ciliated band. 

 But processes of the supra-oesophageal ganglion also pass into this, and 

 in this way the oesophageal connectives arise, whereas the ring-nerve 

 itself, together with the ciliated band, disappears. 



Therefore, according to Kleinenberg's description, here reproduced 

 briefly, the brain and ventral cord appear to have a separate origin, the 

 impulse to which comes through sensory and locomotor organs. 



