378 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



on reversion to the ridge condition in more or less great suppression 

 or diminution of the middle portion of the ridge. 



In the anterior section of the conus four longitudinal ridges 

 develop, situated respectively on the right-hand side (1), dorsally 

 (2), on the left-hand side (3), and ventrally (4). This anterior end 

 of the conus retains as already explained its primitive position and 

 we shall therefore always refer to the four ridges according to the 

 position they have in this undisturbed portion of the conus as Eight, 

 Dorsal, Left and Ventral respectively, the adverb morphologically 

 being understood before the adjective in each case. The right and 

 left ridges make their appearance first and they alone become 

 prominent, forming thin shelf-like structures which project right 

 in to the centre of the cavity so that their edges overlap. For a 

 short distance at the extreme anterior end they become fused 

 together so as to form a continuous septum. The left ridge is 

 comparatively short, tapering off posteriorly, but the right extends 

 back through the anterior and middle section of the conus. At the 

 point of flexure between middle and posterior sections there is a 

 break during early stages but later on the ridge becomes continuous 

 with a portion of ridge which projects from the ventral wall of the 

 posterior section of the conus. There is no reason to doubt that 

 this is really part of the same morphological structure as that with 

 which it is in line in the anterior section of the conus and we shall 

 therefore term it the posterior portion of the right ridge. The 

 whole of this right ridge forms what is often called the spiral valve 

 of the conus. 



The dorsal and ventral ridges of the anterior section of the 

 conus are later than the lateral ridges in making their appearance 

 and soon disappear again. The ventral ridge is especially feebly 

 developed. 



In the posterior section of the conus the right ridge — now ventral 

 in position — is alone well developed. The other three appear as 

 rudiments, they are at no time prominent and they become resolved 

 into vestigial pocket valves which may still be detected in the 

 adults. This latter fact justifies the conclusion already reached 

 that during the ancestral history of the Lung-fishes a stage was passed 

 through during which the conus was provided with longitudinal 

 rows of functional pocket valves, in other words that the primitive 

 ridges seen in the conus of the modern Lung-fish are revertive 

 rather than persistent.' 



Valves of the Heart. — The sinu-auricular opening is guarded 

 on the right side by a valve. This develops out of the inpushed 

 fold of the cardiac wall in the constriction between sinus and 

 atrium. The atrioventricular opening is guarded by a highly 

 characteristic bevelled plug (Fig. 180, AV.p) which when the 

 ventricle contracts is pulled downwards so as completely to occlude 

 the opening. Developmentally this plug arises as a thickening of 

 the atrioventricular ridge. This ridge, which, as already indicated, 



