358 ORR. ' [Vol. I. 



counted for by this latter theory, but the twist is only necessitated 

 by the cranial flexure, and all traces of it disappear in later 

 stages. 



Even the combination of these two explanations does not 

 account altogether satisfactorily for the peculiarities of the cranial 

 flexure. While they perhaps play a considerable role, there 

 seems to be another primary element. The probabilities deduced 

 from our knowledge of the facts point strongly to a primitive 

 paired condition of the central nervous system. This is indi- 

 cated by the facts which have previously been referred to, by 

 deductions from the principles of phyllogeny, and by the paired 

 condition of the adult cerebro-spinal system. This paired con- 

 dition has been a recognized principle in nearly every theory 

 of the origin of vertebrates.^ It is best illustrated in Bombi- 

 nator, where there are the two bands of thickened epiblast run- 

 ning dorso-laterally along the back, spreading apart in the head, 

 and uniting anteriorly in a graceful curve. In all cases where 

 we find a nervous system originating in the epidermis, and then, 

 by sinking into the body, removing itself entirely from the ex- 

 ternal surface, we are forced to conclude that the primary motive 

 is to protect the increasing delicacy of the nervous organ from 

 the rough stimulus of contact with external objects. There is 

 no reason to suppose a different primary motive for this process 

 in vertebrates. The thinner median epiblast connecting the 

 thickened bands is carried inward with the thickened parts, — 

 probably owing to its relatively small size and its intimate as- 

 sociation with the commissures connecting the lateral neural 

 parts. The tendency of the lateral parts of the epiblast to unite 

 above the neural parts is such that they carry with them toward 

 the median line the lateral edges of the neural thickenings. 

 Thus is formed first the medullary groove, to which so much at- 

 tention has been paid, and finally the central canal. Why this 

 canal should have remained open is utterly unknown, but it 

 probably serves, or served, only a secondary purpose. During 

 the period in which these changes take place the neural thick- 

 enings are the densest and most unyielding of the tissues of the 



^ Ihibrecht (" Quar. Jour, of Mic. Sci.," March, 18S7) describes a small median 

 dorsal nerve in nemertines, and suggests its homology with the spinal cord. In view 

 of the bilateral symmetry of the spinal cord, the attempt to derive it from a nerve so 

 small and specialized seems hardly an advance toward the solution of the problem. 



