201 



as hypoblastic or mesoblastic: however the point is of little real 

 significance. 



The later development of the heart follows closely the Ampibian 

 plan. The most striking peculiarity is the almost complete absence 

 of valves throughout development. The auriculo- ventricular cushion 

 which replaces physiologically the usual auriculo-ventricular valves is 

 the only valvular structure indicated : this develops early as a rod of 

 connective tissue which soon is converted into cartilage. Correlated 

 with the very late' formation of the pulmonary system, the auricle 

 shows no sign of division for a very long period: it is entirely un- 

 divided nine weeks after the time of hatching. 



The branchial arteries develop similarly to those of the Ganoids 

 and Amphibia. There are five complete aortic arches located in the 

 hyoid and branchial arches. Each of these is formed from three 

 originally separate elements : these are, a diverticulum from the dorsal 

 aorta, a diverticulum from the ventral aorta and a lacunar space in 

 the arch. These elements fuse forming single continuous trunks connecting 

 ventral and dorsal aortae {ao. Fig. 1 A). Later the hyoidean vessel 

 loses its connection with the dorsal aorta and assumes the relations 

 of a hyomandibular artery exactly similar to that of the Urodela: 

 later it wholly degenerates {h.m. Fig. 1 C) and is not represented in 

 the adult. These primarily continuous aortic arches shortly become 

 replaced by a single afferent and single etierent branchial artery in 

 each arch {a., e. Fig. 1 B) and finally a second efferent artery develops 

 posterior to the afferent vessel (Fig. 1 C). Thus the double efferent 

 branchial artery of Ceratodus must be regarded as a condition parallel 

 with the Elasmobranch and no comparison can be drawn in this 

 respect between the adult Ceratodus and the embryo shark. 



The anterior carotid arteries whose relations in the adult are so 

 peculiar, develop as the anterior prolongations of the lateral dorsal 

 aortae {a.c. Fig. 1) precisely as in all Amphibia and Ganoids. This 

 condition is retained until the young fish is eight or nine weeks old 

 when the anterior carotid artery separates entirely from the dorsal 

 aorta and soon effects a new connection with the vessels of the hyoid 

 arch (hy. Fig. 1 C) : the root of the artery then remains as the posterior 

 carotid artery (p.c. Fig. 1 C). In the light of these embryological 

 facts certain comparisons based upon anatomical relations may be 

 stated as follows: 



a) The similarity between the carotid arteries of Elasmobrauchs 

 and Ceratodus is a parallelism. The connection between the anterior 

 carotid and hyoidean vessels, so charactei'istic of the Elasmobranch, 



