MUSEUM OF COMPAKATIVE ZOOLOGY. 207 



halten ergibt sich jedoch noch eiu anderes bedeutungsvolleren Moment, 

 jeues, namlich, welche deu von der Chorda durchsetzten Abschnitt des 

 Crauiums in gleichen oder doch zunachst abnlicheu Beziehungen zeigt, 

 wie sie die Wirbelsaule zur Chorda besitzt, so das darauf eine Vergleich- 

 uug jenes Abschnittes des Craniums mit einem Abschnitte der Wirbel- 

 saule sich stUtzeu kaun." And further, " Aus der vollen Wiirdigung 

 dieser Beziehung von Gehirn und ISTerven des hinteren Abschnittes 

 ergibt sich der offene Gegensatz zum Vorderen Schadelraum, der von 

 dem hinteren sehr verschiedene Gehirntheile umschliesst und ebeuso in 

 den ihn vei'lassendeu Nerven keinerlei Gemeinsamkeit mit deu von 

 Spinaluerven ableitbaren hinteren Nerven wahi-nehmen liisst. Die 

 Resultate der Vergleichung der einzelneu Abschnitte des Binnenraumes 

 am ausgebildeten Cranium sind somit mit der Priifung der Sonderuugs- 

 vorgange bei der Entstehung des Knorpelcraniums im Einklange." 



To which if we add the weight of evidence afforded by the study of 

 the vascular arrangements described in a previous paragraph, we have 

 increased reason for the separation of the prechordal from the chordal 

 section of the cranium. For with this addition there is not a single im- 

 portant structure entering into the composition of the head which does not 

 shoio traces of the originally distinct separation of these regions, 7iow so 

 closely united among cdl the higher vertebrates. 



Important in this connection is the relation of the chorda in Bom- 

 binator igneus figured by Goette, loc. cit., Taf. IX. Figs. 164, 165, and 

 166, Taf. XV. Figs. 283 and 284, in which the author found the chorda 

 dorsalis lying below the cartilaginnous cranium, although in intimate 

 contact with it. After its degradation, which takes place in an early 

 stage of development, this portion of the chorda is converted into a 

 (keel-shaped 1) calcareous crust, projecting from the ventral surface of 

 the cranium. It is evident that in such a case the dorsal aorta could 

 not becotpe enclosed in the cartilaginous cranium, and in this fact we 

 may have an explanation why a remnant of the aorta is not more 

 uniformly found among the Elasmobrauchs in general. It is important 

 to bear in mind the condition of the head region before the cartilaginous 

 cranium has been formed. In such a primitive animal, or at a cor- 

 responding stage in a n\ore advanced form, the notochord, dorsal (and 

 possibly ventral) aorta, digestive tract, and nervous cord extend through 

 the head region, all parallel to the long axis of the body, and held in 

 place by the connective tissue lying between them and whatever mus- 

 cular and skeletal structures may be present. Branches from the ner- 

 vous axis or the aorta easily find their way through the yielding tissue 



