382 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



As development goes on the wall of the conus becomes changed 

 in histological character, its striped muscles become replaced by- 

 smooth and in general it takes on the ordinary features of arterial 

 wall so that it resembles a portion of the ventral aorta rather than 

 of the heart. As may be seen in a living embryo this histological 

 change is accompanied by a physiological one, for the rhythmic 

 contractions of the heart are seen now to extend forwards as far as 

 the anterior limit of the striped muscle but no farther. Altogether 

 the superficial appearance is just as if the ventral aorta (" truncus 

 arteriosus ") were extending backwards at the expense of the conus, 

 and the word truncus is frequently used to include the whole as far back 

 as the limit for the time being of the smooth non-striated muscular 

 wall. It must however not be forgotten that in the strict morpho- 

 logical sense all that part of the heart is conus which corresponds to 

 the conus of Zepidosiren. The special criterion which identifies it is 



Fig. 181. — Diagrammatic transverse sections through conus of Lacerta (A) and Gallus 

 (B) to show the endocardiac ridges and the pocket valves. 



1 } morphologically right ridge ; 2, dorsal ; 3, left ; and 4, ventral, a and ft, problematical ridges 

 discussed in text ; p, pulmonary cavity ; S, main systemic ; Ls t left systemic cavity. 



the appearance of double flexure or the resultant spiral coiling during 

 its development. The muscular coating, so characteristic a feature in 

 the vertebrates below the Amniota, is associated with a definite 

 type of functional activity : in the Amniota that type of functional 

 activity has disappeared and with it the characteristic type of wall. 



The ventral aorta is in its hinder portion, where it becomes 

 continuous with the front end of the conus, divided x into a dorsal 

 (pulmonary) and a ventral (aortic) cavity by a horizontal septum 

 and this is prolonged backwards along the wall of the conus by the 

 right and left ridges. Of these the right is very large, it projects 

 across the lumen and gradually fuses with the left ridge (Fig. 181, A). 

 This ridge (3) is low and double and it is with its dorsal portion, 

 i.e. the portion next the dorsal ridge, that the fusion takes place. 

 By the spreading backwards of this process of fusion of the right 

 and left ridges the horizontal septum of the ventral aorta becomes 

 prolonged back as a septum in the conus — no longer horizontal 



1 See below, p. 393. 



