2()2 



77//; MEDULLARY CANAL. 



so that the visceral region of the head is longer in the lower Vorte- 

 brata than the neural region, and is dorsally overlapped by the 

 anterior part of the spinal cord and the anterior muscle-plates {vide 

 fig. 47). 



On the above view the posterior part of the head must have been 

 originally composed of a series of somites like those of the trunk, but 

 in existing Vertebrata all trace of these, except in so far as they are 

 indicated by the visceral clefts, has vanished in the adult. The cranial 

 nerves however, especially in the embryo, still indicate the number 

 of anterior somites ; and an embryonic segmentation of the mesol)last 

 has also been found in many lower forms in the region of the head, 

 giving rise to a series of cavities known as head-cavities, enclosed by 

 mesoblastic walls which afterwards break up into muscles. These 

 cavities correspond with the nerves, and it appears that there is a 

 prsemandibular cavity corresponding with the third nerve (fig. 193, Ipp) 



and a mandibular cavity {2pp) and a 

 cavity in each of the succeeding vis- 

 ceral arches. The fifth nerve, the 

 seventh nerve, the glossopharyngeal 

 nerve, and the successive elements of 

 the vagus nerve correspond with the 

 posterior head-cavities. 



The medullary canal. The ge- 

 neral history of the medullary plate 

 seems to point to the conclusion that 

 the central canal of the nervous system 

 has been formed by a groove having 

 appeared in the ancestor of the Chor- 

 data along the median dorsal line, 

 which caused the sides of the nervous 

 plate, which was placed immediately 

 below the skin, or may perhaps at 

 that stage not have been distinctly 

 differentiated from the skin, to be bent 

 upwards ; and that this groove sub- 

 sequently became converted into a 

 canal. This view is not only sup- 

 ported by the actual development of 

 the central canal of the nervous system 

 (the types of Teleostei, Lepidosteus 

 and Petromyzon being undoubtedly 

 secondary), but also (1) by the pre- 

 sence of cilia in the epithelium lining 

 the canal, probably inherited from cilia 

 coating the external skin, and (2) by 

 the posterior roots arising from the extreme dorsal line (fig. 194), 

 a position which can most easily be explained on the supposition 

 that the two sides of the plate, from which the nerves originally 



Fio. 193. Tkansveese section 



THROUGH THE FRONT PART OF THE 

 HEAD OF A YOUNG PrISTIURUS EMBRYO. 



The section, owing to the cranial 

 flexure, cuts both the fore- and the 

 hind-brain. It shews the prteman- 

 dibular and mandibular head-cavities 

 lj)p and 2pp, etc. 



/6. fore-brain; Liens of eye; m. 

 mouth; pt. upper end of mouth, 

 forming pituitary involution ; lao. 

 mandibular aortic arch ; 1pp. and 

 2pp. first and second head-cavities; 

 Ivc. first visceral cleft; V. fifth 

 nerve ; aim. ganglion of auditory 

 nerve; VII. seventh nerve; aa. dor- 

 sal aorta ; acv. anterior cardinal vein ; 

 ch. notochord. 



