208 BULLETIN OF THE 



to their destination.* So long as the intervening spaces remain filled 

 with loose connective tissue, all these spaces are eminently vascular and 

 h-inphatic. The need of greater strength and power of resistance in the 

 slceletal axis calls for a solidification in and about the notochord, and 

 upon the extent of the hardened area depends the nature of the enclos- 

 ures. Usually, of course, the notochord forms the centre of the solidi- 

 fied tract, but we have just seen that it may lie on the ventral border 

 of this tract. The structures transverse to tlie axis are partly enclosed 

 in the solidified tissue, — the proximal portions in the case of the nerves, 

 the proximal or distal in that of the blood and lymph vcssels.t 



The e]uestion of the homology of the carotid arteries has been touched 

 upon by many morphologists, and although the subject has never at- 

 tracted any very great attention, several explanations have been proposed 

 at various times. The usual one found in our text-books on compara- 

 tive anatomy and embryology was the result of determinations made by 

 the earlier embryologists, Bischoff, Rathke, and othei'S, of the ontogeny 

 ' of the vascular system in mannnalian and other embryos. 



Ivolliker's " account contains the whole matter in clear and concise 

 form, and I shall quote his words as a statement of the generally accepted 

 views. He says (loc. cit., p. 915) : "Die erste Form derselben (i. e. die 

 Arterien) die gleich nach der Entstehung des Herzens und wahrend der 

 Dauer des Kreislaufes im Fruchthofe getroffen wird, ist die (Fig. 560. 1) 

 dass das Herz vorn einen Truncus arteriosus entsendet der nach kurzen 

 verlaufe in zwei Ai'cus Aortse sich spaltet, die in der Wand des Kopfdarm- 

 hohle bogenfbrmig nach der Gegend der spateren Schjidelbasis und dann 

 langs dieser convergirend nach hinten laufen, um anfjinglich getrennt 

 von einander als doppelte Aorta descendentes zu enden und spater unter 

 einander zu unpaaren Aorta zu verschmelzen. Sowie die Kiemenbogen 

 . . . hervortreten, zeigf sich, dass der Anfang der Aortenbogen in den 

 ersten Kiemenbogen liegt, sowie dass audi fiir die folgenden Kiemen- 



* These are the cranial and spinal nerves, the afferent and efferent branchials, 

 and musculo-spinal arteries for the most part, all of which do not run parallel to 

 tlie Inng axis. 



t The hlood-vessels are affected most by this process, and all except the im- 

 portant trunks rapidly atrophy, leaving as a last trace a fibrous cord imbedded in 

 the solid cartilage. 



1** fvolliker, A. Entwick. des Menschen, etc. Leipzig, 1879. 



1^ Eathke, H. Entwickelungsgeschiclite der Natter. Konigsberg, 1839. 



2^ Rathke. Ueber die Entwickelung der Arterien welche bei Saugethieren von 

 den Bogen der Aorta ausgehen. Arch, fur Anat. imd Physiol., 1843. 



21 Kathke. Entwickelungsgeschichte der Wirbelthiere. Leipzig, IBGl. 



