380 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



canal and of the conus never forms a spongework but remains as a 

 compact layer of muscle. In the case of the conus this muscular 

 coat is very feebly developed in the middle and cephalic portions — ■ 

 in which fact we may probably recognize a degenerative feature see- 

 ing that in the Elasmobranch the conus musculature is well developed 

 right to its frout end. The meshes of the ventricular spongework, as 

 development goes on, come to spread somewhat round the auricular 

 canal and round the ventricular end of the conus, so that each of 

 these structures has the appearance of being drawn into the ventri- 

 cular cavity. 



Saueopsida. — The most exhaustive account of the development 

 of a Sauropsidan heart is that dealing with Lacerta by G-reil (1903) 

 and upon it the following description is based. In its early stages 

 the heart passes through the familiar tubular form and becomes bent, 

 first bulging in a simple curve towards the right and then assuming 

 a double S-like curvature just as in the Elasmobranch. About stage 

 17-18 the constriction of the heart into sinus venosus, atrium, 

 ventricle and conus becomes apparent — the three last mentioned 

 chambers bulging outwards between the grooves which limit them. 

 The atrial portion does not in these early stages take up the purely 

 dorsal position seen in the Elasmobranch or Lung-fish but remains 

 for a time well to the left. 



The conus, in its early stages, much reduced in relative size as 

 compared with that of the Elasmobranch, undergoes a marked increase 

 in length, which causes it to assume a bayonet-shaped curvature in 

 which we may see a reminiscence of the sharp double flexure seen in 

 the conus of the Lung-fish. In the Lizard however this curvature 

 of the conus is merely temporary. As development goes on the 

 increase in length of the conus instead of being more pronounced 

 than that of the heart as a whole becomes less so with the result 

 that between stages 21 and 26 the anterior flexure of the 1 

 conus becomes pulled out and replaced by a right-handed spiral 

 twist. 



Development of Septa. — The septation of the heart is inaugurated 

 by the appearance of localized proliferation of the endocardiac lining. 

 In the auricular canal, which runs in an antero-posterior direction 

 rather than dorsi-ventrally, owing to the atrium lying anterior to the 

 ventricle instead of dorsal to it as was the case in the Lung-fish, 

 there develop two endocardiac cushions, one dorsal (posterior), the 

 other ventral (anterior). Of these the ventral or anterior one which 

 was not apparent in Lepidosiren is well developed and is continued 

 as an endocardiac ridge round the anterior (headward) wall of the 

 atrium on to its roof (compare Fig. 183, C, at.s). As development 

 goes on this projects more and more prominently into the cavity of 

 the atrium and forms the main part of the septum between the two 

 auricles. By about stage 26 it has grown half-way across the atrial 

 cavity, and by about stage 29 it reaches the auricular canal. While in 

 a'sense the atrial septum is now complete it is not so physiologically 



