THE DIENCEPHALON OF ARACHNIDS AND VERTEBRATES. 6 1 



which the entire brain has slipped forward, underneath the skin, arid in part to 

 this backward growth of the rostrum, the stomodaeum becomes divided into two 

 sections, an inner one passing through a narrow opening between the crura to the 

 enteron; the other extending backward, over its outer surface, to the mouth. 

 The outer section is dilated in most arachnids to form a large chamber or sucking 

 stomach. It is merely a matter of terminology whether we call the mouth the 

 opening of the original infolding, leading directly through the brain, or the opening 

 which lies much farther back beneath the overhanging rostrum. In the verte- 

 brates, both the original infolding and its secondary extension may be recognized. 



As we have shown elsewhere, the closing of the primitive oesophagus was due 

 to several factors, among which were: the crowding together of the cranial neuro- 

 meres; the increasing size of the palial fold; the backward growth of the rostrum 

 and optic ganglia along the anterior median line; and the deepening of the median, 

 neural groove by the precocious thickening of the lateral cords. These condi- 

 tions lead to the infolding of the entire neuron at an early embryonic period, and 

 to its complete separation from the overlying ectoderm. Thus, not only were the 

 eyes enclosed within the brain chamber, but the passage way for the stomodaeum 

 first became greatly constricted, and then the opening into it was covered over by 

 the neural crests and optic ganglia, thus forever closing the entrance to the enteron 

 from that direction. 



The several processes seen in the arachnids, in vertebrate embryos, are blended 

 and abbreviated into a simple marginal overgrowth and an axial depression 

 of the medullary plate. The chamber thus formed over the cheliceral neuromere 

 then becomes the third ventricle; the epithelium of the extra-neural part of the 

 stomodaeum merging with, and forming a part of, the epithelial lining of the ad- 

 jacent cavities. The primitive stomodaeal infolding may still be seen in the 

 amphibia, as a minute pit in the middle of the procephalic lobes, near their anterior 

 margin. (Fig. 46.) This pit lies in the position of the future infundibulum and 

 appears to deepen, giving rise to it. The epithelium forming the floor of the 

 depression is continuous with the epithelial layer that covers the inner surface of 

 the adjacent brain cavities, and represents the deeper end of the stomodaeum, now 

 converted into the membranous saccus vasculosus. 



From the posterior lateral walls of the infundibulum, two rounded ganglionic 

 lobes are evaginated, the lobi inferiori. They correspond with the lateral stomo- 

 daeal ganglia of the arachnids. Like them, they have direct nervous connections 

 only with the adjacent epithelial sac (stomodaeum), although the nerve centers 

 themselves are of considerable size. 



According to Johnston, the epithelial wall of the saccus contains large spindle- 

 shaped, sensory cells, bearing a tuft of cilia which projects into the cavity of the 

 tube (ventricle). From them arise nerve fibers that help form a nerve plexus 

 over the outer surface of the sac. The afferent and efferent fibers form two 

 lateral symmetrical tracts which run through the corpus mammalare to the ventral 

 part of the thalamus. 



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