vi HEAET OF BIEDS 387 



of its development the ventricular portion of the heart undergoes a 

 certain amount of rotation, the right ventricle becoming displaced 

 somewhat towards the left side, ventrally to the left ventricle. The 

 result of this is to undo to a small extent the spiral twist of the 

 conus. 



The conus of the Fowl develops typical endocardiac ridges, here 

 however only three in number, and the individual ridges retain a 

 more nearly primitive condition in that in early stages they are not 

 so completely divided into two distinct rudiments, while in later 

 stages two of the three are obviously continuous. One of these 

 ridges is clearly the morphologically Eight. Here again it is much 

 enlarged (Fig. 181, B, 1) and grows right across the cavity to form a 

 complete septum between the pulmonary (p) and systemic (#) 

 portions of the cavity. 



Eegarding the identity of the two other ridges there is some 

 doubt. They are identified by Greil as the Dorsal (2) and Left (3) 

 while the Ventral (4) is supposed to have disappeared. It appears 

 to the present writer however that the possibility should be con- 

 sidered whether they do not together represent the Left ridge, with 

 which in Lacerta the free edge of the enlarged Eight ridge comes 

 in contact and which in the latter animal shows an incipient 

 division into two parts by a longitudinal groove. 



The septum formed by the enlarged Eight ridge follows a spiral 

 course, its line of insertion being indicated by a spiral groove on 

 the outer surface of the conus. In the Bird this groove gradually 

 deepens into a slit which splits the septum into two halves and as a 

 consequence divides the conus into two separate vessels which 

 course spirally round one another — the roots of the pulmonary 

 arteries and the systemic aorta respectively. No vestige of a 

 septum subdividing the aortic cavity has so far been described. 



Valves. — Pulmonary artery and systemic aorta are each 

 provided with three pocket-valves at their ventricular end. These 

 arise in the manner indicated in Fig. 181, B. Each vessel receives 

 a valve split off from the enlarged Eight ridge. The pulmonary 

 and the systemic cavities receive further a valve split off from the 

 endocardiac thickenings marked a and i respectively. Following 

 Greil these would be attributed to the Dorsal and Left ridges, while 

 accepting the alternative interpretation suggested above they would 

 both be referred to the morphologically Left ridge. There remains 

 a third pocket-valve in each cavity. That in the systemic cavity 

 no doubt represents the otherwise missing Ventral ridge, while if 

 a and b together represent the Left ridge then the third pocket- 

 valve of the pulmonary cavity would represent the Dorsal ridge. 

 As there is no reason to doubt the reliability of a pocket-valve as 

 evidence of a once existing endocardiac ridge we should' be driven — 

 if we reject the explanation here suggested — to assume the former 

 existence of an additional ridge between the Eight and the Dorsal 

 and there seems no justification otherwise for doing this. 



