CHAPTER LVL 



THE NERVOUS SYSTEM. 



It will be remembered that the nervous system begins its career in 

 the embryonic state by the ectoderm of the gastrula becoming flat on the 

 dorsal surface of the embryo. This flat portion, called the neural plate, 

 extends practically the entire length of the embryo. It is slightly 

 broader at the head end than at the caudal end. The two edges of the 

 neural plate become raised slightly and finally meet in the mid-line of 

 the dorsal surface to form a tube. The closing of the tube begins at the 

 head end and gradually extends backward until the tube has become 

 completely closed. (Fig. 262). 



The neural plate, consisting of ectoderm and folding as it does, 

 causes the interior lining of the tube to be ectoderm. This is a point 

 of considerable value in making it easier to understand various 

 structures, such as the development of the eye. It is also well to re- 

 member here that the various sense organs, or special organs of sense, 

 as they are often called, have to do with such things as touch, sound, 

 taste and light, whose stimuli come first to the exterior part of the body. 

 In the lower animal forms, such as the earthworms, there are no definite 

 eyes, and yet, light rays, when thrown upon any part of the earthworm's 

 body, cause it to move out of such light, showing that the animal is 

 sensitive to these light rays even though there be no organ developed 

 by which any one particular spot is specialized to receive more im- 

 pressions than another part. 



Now, just as the complete digestive tract develops from a straight 

 tube by inpushings and outpushings, so the greater portion of the 

 nervous system has developed in a similar manner from the single nerve 

 tube that has just been discussed. 



One of the explanations given as to why the nervous system de- 

 velops in the way it does from the ectoderm and on the dorsal surface of 

 the embryo, is that remote ancestors of the vertebrates may have spent 

 their years upon the ocean-bottom, causing the ventral surface of the 

 body to lie in contact with the ground substance and thus serve as a 

 protection from attack, while the upper part of the body came in contact 

 with substances and animals inimical to it. These vertebrate ancestors 

 thus needed a sense-perception organ for protective and nutritive pur- 

 poses. Interesting as this may be, it must be admitted that one of the 

 great difficulties with which biologists have had to deal is the fact that, 

 in the invertebrates, the nervous system lies upon the ventral surface. It 



