554 



NA TURE 



[October 8, 1896 



pointed out in my previous papers,' every part of the central 

 tube of the vertebrate nervous system corresponds absolutely, 

 both in position and structure, with the corresponding part of 

 the alimentary canal of the arthropod, and the nervous material 

 which is arranged round this epithelial tube is identically the 

 same in topographical position, in structure, and in function as 

 the corresponding parts of the central nervous system of an 

 arthropod. 



Especially noteworthy is it to find that the pineal eye (PN), 

 with its large optic ganglion, the ganglion habenulie |(}HR), 

 falls into its right and appropriate place as the right median eye 

 of such an animal as Limulus or Euryplerus. In the following 

 table I will shortly group together the evidence of the anatomical 

 test. 



A. Coincidence of Topographical Position. 



Limulus and its Allies. Ammoccktes and Vertebrates. 

 Alimentary Canal : — 



1. Cephalic stomach. Ventricles of the brain. 



2. Straight intestine, end- Spinal canal, ending by means of 



ing in anus. the neurenteric canal in the anus 



3. CEsophageal tube. Median infundibular tube and 



saccus vasculosus. 

 Nervous System : — 



1. Supranesophageal gan- Brain proper, or cerebral hemi- 



glia. spheres. 



2. Olfactory ganglia. Olfactory lobe. 



3. Optic ganglia of the Optic ganglia of the lateral eyes. 



lateral eyes. 



4. Optic ganglia of the Ganglia habenula;. 



median eyes. 



5. Median eyes. Pineal eyes. 



6. (Esophageal commis- Crura cerebri. 



sures. 



7. Infra-cesophageal or Hind brain, giving origin to the 



prosomatic ganglia Ilird, IVth, and Vth cranial 



giving origin to the nerves, 

 prosomatic nerves. 



8. Mesosomatic ganglia. Medulla oblongata, giving origin 



giving origin to the to the Vllth, IXth, and Xth 

 mesosomatic nerves. cranial nerves. 



9. Metasomatic ganglia. Spinal cord. 



B. Coincidence of Structure and Physiological Function. 

 (i) The simple non-glandular epithelium of the nerve tube 

 coincides with the simple non-glandular epithelium of the 

 alimentary canal, ciliated as it is in Daphnia (Hardy and 

 McDougall, Proc. Camh. Philos. Soc, vol. viii., 1893). 



(2) The structure and function ol the cerebral hemispheres, 

 olfactory lobes, and optic ganglia closely resemble the corre- 

 sponding parts of the supra-resophageal ganglia. 



(3) The structure of the right pineal eye, with its nerve end- 

 cells and rhabdile<, is of the same nature as that of a median 

 arthropod eye. 



(4) The structure of the right ganglion habenul* is the same 

 as that of the optic ganglion of the median eye. 



(5) The region ol the hind brain, like the region of the infra- 

 oesophageal ganglia, is concerned with the co-ordination of 

 movements. 



(6) The region of the medulla oblongata, like the mesoso- 

 matic region of Limulus and its allies, is concerned especially 

 with the movements of respiration. 



(7) The centres for the segmental cranial nerves resemble 

 closely in their groups of motor cells and plexus sub-tance the 

 centres for the prosomatic and mesosomatic nerves, with their 

 groups of motor cells and reticulated substance (Punkt- 

 .Substanz). 



3. The third test is the ontogenetic test. The theory must be 

 in harmony with, and be illustrated by, the embryonic develop- 

 ment of the central nervous system. Such is the case, for we 

 see that the nerve tube arises as a simple straight tube opening 

 by the neurenteric canal into the anus, the anterior part of the 

 tube, i.e. the cephalic stomach region, being remarkably dilated ; 

 the anterior opening of this tube, or anterior neuropore, is con- 

 sidered by most authors to have been situated in the infundibular 

 region. 



Next conies the formation of the cerebral vesicles, indicating 

 embryologically the constricting growth of nervous material 



1 Gaskell, Jon 

 Physiol., vol. x , 



ut rityshl., vol xxm.. 1888 ; Jou 

 . xii., 1S89; Q.y. o/Micr. .'^ci., 18; 



NO. 1405, VOL. 54] 



outside the cephalic stomach. First, the formation of two 

 cerebral vesicles by the growth of nervous material in the 

 position of the ganglia habenulrc, posterior commissure, and 

 Meynert's bundle, i.e. the constricting influence of commissures 

 between the optic part of the supra- ri-sophageal ganglia and the 

 infra-a:;sophageal ganglia : then the formation of the third 

 cerebral vesicle by the constricting influence of the IVth nerve 

 and commencing cerebellum. Subsequently the first cerebral 

 vesicle is divided into two parts by another nerve commissure — 

 the anterior commis.sure, i.e. by nerve material joining the supra- 

 nesophageal ganglia. Further, the embryological evidence shows 

 that in the spinal cord region the nerve masses are at first most 

 cons|)icuous vent rally and laterally to the original tube, such 

 ventral masses being early connected together with the strands 

 of the anterior commissure : ultimately, by the growth of nervous 

 material dorsalwards, the dorsal portion of the lube is compressed 

 to form the posterior fissure and the substantia Rolandi, the 

 original large lumen of Ihe old intestine being thus reduced to 

 the small central canal of the adult nervous system. Finally, 

 this nerve tube is formed at a remarkably early stage, just as 

 ought to be the case if it represented an ancient alimentary 

 canal. 



The ontogenetic test appears to fail in two points : — 



( 1 ) That the nerve tube of vertebrates is an epiblastic tube, 

 whereas if it represented the old invertebrate gut it ought to be 

 largely hypoblastic. 



(2) The nerve tube of vertebrates is formed from the dorsal 

 surface of the embryo, while the central nervous system ot 

 arthropods is formed from the ventral surface. 



With respect to the first objection, it might be argued, with a 

 good deal of plausibility, that the term hypoblast is used to 

 denote that surf;ice which is known by its later development to 

 form the alimentary canal ; that in fact, as Heymons (" Die 

 Embryonalentwickl. v. Dermapteren u, Orthopteren," Jena, 

 1895) has pointed out, the theory of the germinal layers is not 

 sufficiently well established to give it any phylogenetic value. 

 It is, however, unnece.ssary to discuss this question, seeing that 

 Heymons has shown that the whole alimentary tract in such 

 arthropods as the earwig, cockroach, and mole cricket, is, like 

 the nerve tube of vertebrates, formed from epiblast. 



Tlie second objection appears to me more apparent than real. 

 The nerve layer in the vertebrate, as soon as it can be dis- 

 tinguished, is always found to lie ventrally to the layer of 

 epiblast which forms the central canal. In the middle line 

 of the body, owing to the absence of the mesoblast layer, the 

 cells which form the notochord and those which form the central 

 nervous system form a mass of cells which cannot be separated 

 in the earlier stages. The nerve layer in the arthropod lies 

 between the ventral epiblast and the gut ; the nerve layer in the 

 vertebrate lies between the so-called hypoblast {i.e. the ventral 

 epiblast of the arthropod) and the neural canal (i.e. the old gut 

 of the arthropod). The new ventral surface of the vertebrate in 

 the head region is not formed until the head fold is completed. 

 Before this time, when we watch the vertebrate embryo lying on 

 the yolk, with its nervous system, central canal, and lateral 

 plates of mesoblast, we are watching the embryonic representa- 

 tion of the original Limulus-like animal ; then, when the lateral 

 plates of mesoblast have grown round, and met in the middle 

 line to assist in forming the new ventral surface, and the head 

 fold is completed, we are watching the embryonic representation 

 of the transformation of the Limulus-like animal into the scorpion- 

 like ancestor of the vertebrates. 



In the Arthropoda, the simple epithelial tube which forms the 

 .stomach and intestine is not a glandular organ, and we find that 

 the digestive part of the alimentary tract is found in the large 

 organ, the so-called liver. This organ, together with the 

 generative glands, forms an enormous mass of glandular sub- 

 stance, which, in Limulus, is tighily packed round the whole of 

 the central nervous .system and alimentary canal, along the 

 whole length of the animal (represented in Fig. 4 by the dark 

 dotted substance). The remains of this glandular mass are seen 

 in Ammoc(etes in the peculiar so-called packing tissue around 

 the brain and spinal cord (represented in Fig. 6 by the dark 

 dotted sub.stance). It satisfies the three tests to the following 

 extent : — 



(i) The phylogenetic test. — As we descend the vertebrate 

 phylum, we find that the brain fills up the brain-case to a less 

 and less e.xtent, until finally in Ammocoetes a considerable space 

 is left between brain and brain-case, filled up with a peculiar 

 glandular-looking material, interspersed with pigment, which is 



