948 REPORT—1896. 
B. Coincidence of Structure and Physiological Function. 
1. 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.! 
2. The structure and function of the cerebral hemispheres, olfactory lobes, and 
optic ganglia closely resemble the corresponding parts of the supra-cesophageal 
ganglia. 
© 3, The structure of the right pineal eye, with its nerve end-cells and rhabdites, 
is of the same nature as that of a median arthropod eye. 
4, The structure of the right ganglion habenule is the same as that of the 
optic ganglion of the median eye. 
5. The region of the hind brain, like the region of the infra-cesophageal ganglia, 
is concerned with the co-ordination of movements. 
6. The region of the medulla oblongata, like the mesosomatic 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 substance 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 development 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, ze. 
the cephalic stomach region, being remarkably dilated ; the anterior opening of this 
tube, or anterior neuropore, is considered by most authors to have been situated in 
the infundibular region, 
Next comes the formation of the cerebral vesicles, indicating embryologically 
the constricting growth of nervous material outside the cephalic stomach. First, 
the formation of two cerebral vesicles by the growth of nervous material in the 
position of the ganglia habenulz, posterior commissure, and Meynert’s bundle, 2.e. 
the constricting influence of commissures between the optic part of the supra-ceso- 
phageal ganglia and the infra-cesophageal ganglia; then the formation of the third 
cerebral vesicle by the constricting influence of the [Vth nerve and commencing 
cerebellum. Subsequently the first cerebral vesicle is divided into two parts by 
another nerve commissure—the anterior commissure, z.e. by nerve material joining 
the supra-cesophageal ganglia. Further, the embryological evidence shows that in 
the spinal cord region the nerve masses are at first most conspicuous ventrally 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 tube is compressed to form the 
posterior fissure and the substantia Rolandi, the original large lumen of the 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 caval. 
The ontogenetic test appears to fail in two points :— 
1. That the nerve tube of vertebrates is an epiblastic tube, whereas if it repre- 
sented 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 of 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 surface which is known 
by its later development to form the alimentary canal; that in fact, as Heymons? 
has pointed out, the theory of the germinal layers is not sufficiently well esta- 
blished to give it any phylogenetic value. It is, however, unnecessary to discuss 
* Hardy and McDougall, Proc. Camb. Philos. Soc. vol. viii. 1893. 
* Heymons, Die Embryonalentwichl. v. Dermapteren wu. Orthopteren, Jena, 1895. 
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