vi HEAET OF LACEETA 381 



as secondary perforations have made their appearance in the septum 

 so as to keep the two auricular cavities in free communication. The 

 two endocardiac cushions of the auricular canal become joined 

 together by a bridge of endocardiac tissue which forms the free edge 

 of the auricular septum. This is followed by a complete fusion 

 taking place between the middle parts of the two cushions, so that 

 the atrioventricular opening becomes completely divided into a larger 

 right and a smaller left portion. The, at first, thick mass of tissue 

 which separates these two openings becomes gradually converted into 

 a thin plate, situated in the plane separating atrium from ventricle, 

 and therefore perpendicular to the plane of the atrial septum. This 

 plate is divided sagittally into a right and a left half by its line of 

 attachment to the septum. The free edge of each half is concave 

 and projects freely into the corresponding auriculoventricular 

 opening — forming the mesial or septal valve of that opening. 



In the meantime a new endocardiac cushion develops on what 

 were the right and left sides of the auricular canal. These also 

 become thin flaps and form the lateral auriculoventricular valves. 

 It will be noticed that there have developed round the original 

 atrioventricular opening four proliferations of endocardium — the 

 same number as was found in the conus of the Lung - fish and 

 as will be found in the conus of the Amniota, thus supporting the 

 idea that there are four longitudinal endocardiac ridges potentially 

 present throughout the cardiac tube of the higher Vertebrates though 

 they may become actually apparent only in the conus region. 



In the Lizard the atrioventricular ridge, which was so conspicuous 

 in the Lung-fish, has practically become reduced to the portion lying 

 within the auricular canal — the dorsal (posterior) endocardiac cushion. 

 The ventricle is undivided. 



The conus on the other hand undergoes a complete and somewhat 

 complicated process of septation. This is inaugurated by localized 

 proliferation of the endocardium to form longitudinal ridges. As in 

 the Lung-fish these arise discontinuously there being distinct anterior 

 (headward) and posterior rudiments. Anteriorly the normal four 

 ridges develop, the dorsal and ventral appearing in this case at an 

 earlier stage (17 or 18) than the lateral ones. Towards the ventricular 

 end two ridges first make their appearance in a dorsal (ridge B, 

 Greil) and ventral situation (A, Greil) respectively. Of these the 

 ventral one becomes eventually continuous with the right-hand 

 anterior rudiment. It clearly corresponds with the similarly situated 

 ridge in the hinder portion of the conus of lepidosiren and like it is 

 to be interpreted as the hinder portion of the morphologically right- 

 hand ridge. The ridge vis-ct-vis to that just mentioned, here dorsal 

 in position, would similarly represent the hinder portion of the 

 morphologically left-hand ridge. Later on a small and transient 

 ridge (C, Greil) makes its appearance on the right-hand wall of 

 this hinder portion of the conus and this would represent the 

 hinder portion of the morphologically dorsal ridge. 



