No. I.] MORPHOLOGY OF VERTEBRATE HEAD. 89 



median walls of the anterior head cavities, from which it would 

 seem that they may be segmentaily related to these anterior 

 cavities. 



A glance at the reconstruction (Fig. 8) makes evident the sig- 

 nificance of the blood-space now lying dorsal to the point of the 

 notochord. Originally the aorta, or aortae, lay straight below 

 the chorda, extending as paired vessels to fuse anteriorly in a 

 single wide chamber, from which two or, if we include the 

 arteriae ophthalmicae magna?, three pairs of aortic branches 

 were given off. When the flexure of the chorda occurred, the 

 point of the chorda passed through the aorta, almost severing 

 an anterior portion of the vessel, with one pair of branches from 

 the main aorta, and placing them in their present abnormal 

 position apparently dorsal to the notochord. If the ophthal- 

 micae magnae are to be viewed as the remains of aortic arches, 

 what shall we say of these anterior branches which are well 

 developed when but one complete aortic arch exists, and when 

 the ophthalmicae magnae and the hyoid arteries are represented 

 as small pockets in the wall of the aorta ? Whether or not 

 these vessels ever functioned as aortic arches, and aside from 

 any consideration of imaginary gills they may once have sup- 

 plied, we may surely view them as segmental vessels, giving one 

 further clue to the lost metamery of the head. 



At the stage of development from which Fig. 8 is reconstructed 

 no cardinal veins are to be found, unless a few isolated spaces in 

 the neighborhood of the vagus, facial, and trigeminal ganglia 

 are scattered beginnings of a more definite channel. 



In the next stage that I have figured (Fig. 9), two pairs of 

 aortic arches are complete ; namely, the mandibular and the 

 hyoid. Two posterior pairs of arches may be traced a short 

 distance from the aorta. The condition of the anterior vessels 

 is much the same as represented in Fig. 8, save that a pair of 

 small branches (cd. 1 ) are now seen to arise from the anterior and 

 dorsal wall of the aorta. They may be traced backward a short 

 distance, but soon disappear. These vessels are the first definite 

 rudiments of the cardinal veins. They soon separate from the 

 aorta, taking with them as they part the aortic space posterior 

 to the chorda, with its pair of aortic branches. Fig. 10 repre- 

 sents the completed separation. There are now six aortic arches, 

 and the arteriae ophthalmicae magnae (ppk. m.) are well developed. 



