390 EMBRYOLOGY OF THE LOWER VERTEBRATES Ctt 



continues until tin- ventricle mines t<> lie ventral to the sinus (Fig. 

 184, C) instead of well in front of it as it did originally. The ventricle 

 is nn\\ <in the bail ward side of the atrium which bulges out on each 

 side, its ventral wall fitting closely to the dorsal side of the conns. 

 The sinus becomes marked off from the atrium by a constriction, 

 which deepens most markedly on the right side so that the sinu- 

 atrial opening hecomes displaced towards the left. It is to be noted 

 that during these stages in development the portion of the ventricular 

 wall lying on its concave (anterior) side undergoes relatively very- 

 slight increase in size. The .result is that the ventricle bulges in a 

 tail ward direction and the openings by which it communicates with 

 atrium and conns respectively remain relatively closely approximated 

 together. 



The atrial septum arises as a ridge or fold of endocardium which, 

 as in the Sauropsida, projects into the lumen from the anterior (head- 

 ward) wall. The concave free edge of this grows towards the atrio- 

 ventricular opening while its base of attachment spreads, on the one 

 hand ventrally until it becomes continuous with the anterior atrio- 

 ventricular cushion, and on the other dorsally and backwards, to the 

 left of the sinus opening and to the right of the pulmonary vein 

 opening, till it becomes continuous with the posterior (tailward) 

 atrioventricular cushion. The conspicuous openings in the atrial 

 septum known to exist in adult Urodeles though not in Anura are 

 secondary perforations. 



The conus develops discontinuous ridges. Of these there are 

 four anterior rudiments of which the Dorsal and Ventral develop 

 first and the Right and Left later. Of posterior rudiments only 

 three have been described but that the missing one is at least some- 

 times present is shown by the adult arrangements of the resulting 

 pocket - valves in different Amphibians. Thus four pocket-valves 

 have been observed in the posterior circle in specimens of Siren, 

 Necturus, the Axolotl, and Salamandra (Boas). In the anterior 

 circle cases are known of one of the four valves being reduced in 

 size (Siren, Axolotl, Triton, Salamandra), vestigial (Pipa), or gone 

 entirely (Proteus, Rana). Other cases occur in which an additional 

 valve makes its appearance through one of the original ones becoming 

 split (Necturus). All these variations in the pocket -valves of tl it- 

 adult are of importance in relation to the embryonic ridges which 

 the valves represent. Details will be found in Boas (1882). 



Of the anterior rudiments the right-hand ridge (\) is pro- 

 longed backwards as the spiral fold which projects across the lumen 

 and divides it impeiT.-ei 1\ int.. an aortic and a pulmonary eavity. 

 In some cam the -piral fold apparently makes an abortive attempt 

 lo pursue the eourse of development \\hieli it- went through in the 

 tral ti-h-like Inini. as it segments up int., a m\v nl' little knobs 

 each of \\hieh. \\ e may take it. represents a pocket -val\ e < Triton 

 punctatu* T. ori hitux). In other 00866 (Necturus, Coecilia) the spiral 

 fold has in the adult mmplrti'ly disappeared (Unas, 1882). 



