92 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS. 



line, over the neural surface of the brain; they rest on the latter by two short 

 stalks. In many other phyllopods and arachnids, the optic ganglia occupy a 

 similar position, in that they lie close together, over the neural surface of the 

 brain, behind the hemispheres. In the vertebrates, they permanently occupy 

 this position, and have become united by secondary commissures, one in the 

 habenulae, the other in the optic lobes. (Fig. 308.) 



The most conspicuous commissure in the arthropods and one of the first to 

 appear, is that belonging to the system of stomodseal nerves. Its gangha, one 

 median and two lateral, arise from the walls of the stomodaeum. It is the only 

 commissure originally provided with ganglion cells, and the only one formed 

 primarily across the neural surface of the brain. It has especial relations to 

 nutrition, through its association with the olfactory, swallowing, and taste centers. 

 It ];epresents the primitive cerebellar commissure of vertebrates, where it appears 

 to have had similar relations. 



V. The Neuroccelia . 



Summary. — The transformation of the paired nerve cords of invertebrates 

 into the hollow nerve tube of vertebrates is affected by several independent factors. 

 These factors make their appearance as active forces in the arachnids, and they 

 have already established there the salient features of the vertebrate neuroccelia. 

 These factors are as follows : 



1. The infolding for the middle cord initiates the canalis centralis and the 

 more posterior parts of the brain cavities. 



2. The increasing depth of the median groove, and the increasing thickness 

 of the two cords, brings the median edges of the cords together, and leads to the 

 formation of the neural commissures, which form the rafters over the median 

 groove, and aid in its conversion into a canal. 



3. The formation of the palial overgrowth for the forebrain, and the mar- 

 ginal overgrowths in the hindbrain region, initiates the development of the broad, 

 membrane-roofed ventricles of the whole brain. 



4. The stomodasal infolding, between the forebrain and midbrain, establishes 

 the deep and narrow third ventricle of the diencephalon. 



5. The deep transverse infolding across the very anterior end of the medul- 

 lary plate gives rise to the cavity of the olfactory lobes. 



6. The broad chamber formed by the palial overgrowth, and into which the 

 hemispheres project, establishes the prosencoele, or the first and second ventricles. 



7. The median and the lateral eye ganglia unite above the neural surface 

 of the brain, one forming a partial roof to the diencephalon, and the other, owing 

 to the shape of the ganglion, forming a broad, dome-like covering for that part of 

 the brain chamber known as the mesencoele. 



8. The stomodceal commissure, forced backward by the enlarging optic 

 ganglia, forms the first stage of the narrow arch (cerebellum) over the future 

 metencoele. 



