CHAPTER V. 



MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS. 



Methods. 



In order to better understand the minute structure of the brain, it is essential 

 to first determine the structure of one of the isolated neuromeres of the spinal 

 cord. I have used for this purpose one of the branchial neuromeres of Limulus. 

 But even here the structure is so exceedingly complex that it is possible to work 

 out or to represent but a very small part of it in detail. Our observations on the 

 structure of an invertebrate neuromere will be of special interest, because they 

 are based on the gross anatomy and embryology, upon the minute structure as 

 obtained by several methods of analysis, and upon the physiology. 



For the development of the primitive sense buds and the early embryonic 

 stages of the cord, I have used the scorpion. For the distribution of the fibers 

 and cells, I have used the methylene blue "intra vitam" method on Limuli from 

 2 to 6 inches long; the brain and cord being mounted whole, or sectioned. The 

 adult nervous system has been tested for its physiological reactions, and the dis- 

 tribution of the peripheral nerves has been followed with care. Its minute 

 structure was worked out from sections prepared by several methods, the most 

 satisfactory ones being the usual Golgi method, and von Rath's picro-osmic- 

 platinum chloride mixture, followed by methyl alcohol. 



I. The Branchial Neuromeres in Limulus. 



Development. — In Limulus and in the scorpion, during the late embryonic 

 stages, a thin epithelial overgrowth forms on the lateral margins of the cords, and a 

 deep infolding appears in the median line between them. The lining to the me- 

 dian infolding is in part converted into the epithelium of the canalis centralis, 

 in part into the inner neurilemma, or neuroglia, and into the lemmatochord. 



The lateral cords themselves sink bodily below the surface, without separat- 

 ing off a surface epithelium, and are covered by an ectodermic layer formed from 

 the marginal overgrowth united to the local (interganglionic) remnants of, the 

 middle cord. (Figs. 227,231.) As in the arthropods generally, they lie in a 

 horizontal plane, separated by the median infolding that forms the middle cord. 

 In the vertebrates, similar conditions prevail, except that the median margins of 

 the lateral cords are more deeply infolded than the lateral ones, thus bringing the 

 neurogenic surfaces of the cords face to face. Thus the two lateral cords of the 



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