THE MKDI'LLAHY CAXAL. 



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

 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 mesoblast 

 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 

 proemandibular 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 glossopbaryngeal 

 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 

 TRANSVERSE SECTION plate, which was placed immediately 

 THROUGH THE. FRONT PART OF THE below the skin, or may peril aps at 



HEAD OF A YOUNG PmSTIURUS EMBRYO. ^ ^^ ^ j^ ^^ JJ^^] 



The section, owing to the cranial -,.,, , f ,-, , i_ T_ 



flexure, cuts both the fore- and the differentiated from the skin, to be bent 



hind-brain. It shews the praBinan- upwards ; and that this groove sub- 



dibular and maudibular head-cavities se queutly became converted into a 

 Ipp and 2pp setc. caml ^ yiew Jg not ^ _ 



//;. fore- brain; /.lens of eye; m. , , , i j i r 



mouth; P t. upper end of mouth, ported by the actual development of 

 forming pituitary involution ; lao. the central canal of the nervous system 



mandibular aortic arch ; Ipp. and / th(J t of Teleostei, Lepidosteus 



2pp. first and second head-cavities ; , . \ . ,, 



ire. first visceral cleft; F. fifth and Petromyzoii being undoubtedly 



nerve; aim. ganglion of auditory secondary), but also (1) by the pre- 



nerve; VII. seventh nerve; an. dor- gence o f ' c ii; a j n t ] ie epithelium lining 



sal aorta; acr. anterior cardinal vein; . . . , , . , L . , .,. 



eh. uotochord. the canal, probably inherited trom cilia 



coating the ex'ternal 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 



FIG. 193. 



