130 WALTER HEAPB. 



do not, however, as yet form continuous tubes. From the front 

 end of the aorta on each side a short vessel is given off which 

 lies dorsal to the aorta and immediately below the nervous 

 system ; it does not, however, extend far. There is no commu- 

 nication between the aortse and the heart tubes at this stage. 



At the commencement of Stage h the two tubes of the heart 

 have met at their anterior end, and form a single wide tube for 

 a short distance (figs. 34 and 25), a single pair of aortic arches 

 are formed and the dorsal aortae extend backwards as two 

 separate tubes some distance beyond the last protovertebra ; 

 just before they terminate they give off two vitelline arteries. 



A series of short diverticula project from the aorta dorso- 

 laterally between the somites, and ventrally, below them at 

 this stage and during Stage J (compare figs. 27 — 31 and 52). 



From near the front end of the aortse, a little posterior to 

 the point where the aortic arch runs into it, two internal carotid 

 arteries are projected forwards and extend to the under surface 

 of the optic lobes (fig. 21, i. c. a.) ; while from about the same 

 point two vessels run backwards joined at intervals with the 

 aortse (fig. 24, v. a.) on each side of, and closely applied to, the 

 now closed neural canal. These vessels run back to a point 

 just in front of the first protovertebra and are doubtless the 

 vertebral arteries. 



Stage J shows little alteration ; the heart is still in the form 

 of a straight tube somewhat longer than in Stage h, but without 

 curvature or any sign of a division into chambers ; there is still 

 also only one pair of aortic arches, and two separate aortae are 

 still present throughout the extent of their course. 



A number of small vessels are now given off from the internal 

 carotid arteries, and the aortee in their anterior portion also 

 send short branches into the surrounding tissue. The vessels 

 which I have before described, running backwards on each side 

 the nervous system, are frequently in communication with the 

 aortae, and it is these vessels which appear at this stage to 

 project diverticula into the substance of the walls of the spinal 

 canal {vide above) (fig. 43). 



The vitelline arteries are given off about on a level with the 



