THE NERVOUS SYSTEM 



407 



the surface of the body, especially upon that portion which, 

 through the customary position of the body, is not exposed to 

 such stimuli. Such superficial systems are still found among 

 lower invertebrates ; in sessile radiate forms equally developed 

 on all sides of the projecting rim or upon the tentacles, in free- 

 swimming forms as an apical plate located upon the point 

 which first comes in contact with external objects. That such 

 was also the case with the unknown ancestor of vertebrates 

 is suggested by the embryonic history of the neural tube, for 

 it is formed here by the rolling in of the external dorsal sur- 

 face of the early embryo. This process is inaugurated by the 

 formation of two longitudinal medullary folds, one upon either 

 side of the middle line, and as these are united around the 

 anterior end and diverge posteriorly, they form for a time a 

 figure not unlike that of an ordinary hair-pin. The area en- 

 closed by these, which consists of a strip along the dorsal 

 surface, becomes somewhat sunken, and as the two medullary 

 folds, beginning anteriorly, approach one another, and finally 

 unite, the area becomes the bottom of a trough, and eventually 

 the inner surface of a tube. 



The complete coalescence of the folds and the pinching off 

 of the trough are the final steps in the process, which results 

 in the formation of the neural tube as described above, the 

 anlage of the central nervous system. If we may take this 

 process as a recapitulation of pre-vertebrate conditions, a view 

 sustained by its universality and the reasonableness of the con- 

 clusions, it suggests that the primitive ancestor of vertebrates 

 was exposed to external stimuli mainly over its dorsal surface, 

 a supposition which in its turn suggests a slightly flattened, 

 worm-like form, with the ventral side resting upon the ground, 

 here undoubtedly the ocean bottom. The greater development 

 of the anterior portion of this tube, even from the first, sug- 

 gests a locomotive habit, which would thus favor the anterior 

 end in this regard. As this superficial nervous system became 

 more highly developed, and hence more sensitive, it was pro- 

 tected in the most natural way for such a system, by the for- 

 mation of elevated ridges along its lateral borders, thus form- 



