vi HEAKT OF LACEETA 383 



however but spirally twisted owing to the twisting of the conus 

 already mentioned. Parallel with and preparatory to this process of 

 fusion the distal ridge rudiments spread backwards, pursuing a spiral 

 course and thus making evident the spiral twisting of the conus as a 

 whole. It is interesting to notice that the line of insertion of the 

 ridges, and therefore of the septum formed by their fusion, becomes 

 marked on the outer surface of the conus by a distinct incision — a 

 preliminary step towards the complete splitting of the conus in the 

 plane of the septum which takes place in Birds and in Mammals. 



In addition to the dividing of the cavity of the conus into a 

 pulmonary and an aortic portion in the manner just described there 

 takes place also, in the Lizard^ a splitting of the aortic portion into 

 two parts, corresponding to the right and left halves into which the 

 systemic portion of the ventral aorta is divided. 1 The septum 

 separating these becomes prolonged backwards at its hinder end, on 

 the one hand, into the ventral ridge of the conus (Fig. 181, A, 4) 

 and on the other into a quite similar ridge developed upon the 

 surface of the septum which separates the aortic from the pulmonary 

 (Fig. 181, A, 1). These two ridges facing one another across the 

 aortic cavity gradually extend backwards and undergo fusion just as 

 in the other case so as to form a complete septum dividing the 

 aortic cavity into two (S and l.s). 



In this way then the original conus becomes replaced by a set of 

 three tubes twisted spirally round one another, forming the roots of 

 the two systemic aortae and of the pulmonary artery, still however 

 enclosed in a common wall. 



Valves of the Heart. — The right and left valves which guard 

 the opening from sinus into atrium are formed simply by the 

 exaggeration of the fold of the cardiac wall which delimits these two 

 chambers from one another. The origin of the auriculoventricular 

 valves has already been described. The pocket valves of the systemic 

 aortae and pulmonary artery are derived from the endocardiac ridges 

 of the conus as in the Elasmobranch. According to Langer (1894) 

 the outer valve in each of the three vessels (pulmonary artery, left 

 systemic aorta, right systemic aorta) are derived from the Dorsal, Left 

 and Ventral endocardiac ridges (2, 3 and 4) respectively, while the 

 inner valve in all three is derived from the hypertrophied Eight 

 ridge (1), which with its outgrowth takes part in the formation 

 of all three vessels (Fig. 181, A). 



The question as to whether or not the pocket valve is formed 

 from the extreme ventricular end of the conus ridge, or whether on 

 the other hand a considerable portion of this end of the conus with 

 its contained ridges becomes incorporated in the ventricle as 

 maintained by Langer and Greil does not appear to the present 

 writer to be satisfactorily settled. It is advisable that the point 

 should be re-investigated upon abundant material. 



G-allus. — (Figs. 182 and 183.) The most detailed investigations 

 1 See below, p. 393. 



