376 EMBRYOLOGY OF THE LOWEE VERTEBRATES oh. 



The first peculiarity to be noticed in comparison with the heart 

 of the Elasmobranch is correlated with the fact that the head and 

 anterior trunk region of the embryo are bent downwards and closely 

 applied to the surface of the yolk. As a consequence, the pericardiac 

 space is reduced to a flat chink, the point of entrance and the point 

 of exit of the cardiac tube being in close proximity to one another at 

 its upper end. The result is that the cardiac tube as it grows in 

 length assumes the form of a flattened loop, first V-shaped (Fig. 

 179, A) and later 8-shaped, the apex of the loop being directed in a 

 ventral direction, and the originally posterior (tailward) limb of the 

 loop (at) coming to he to the left of the anterior limb (C). The 

 cardiac tube becomes demarcated into the same set of chambers 

 as in the Elasmobranch — sinus venosus, atrium formed from the 

 posterior limb of the cardiac loop, ventricle formed from the 

 apical portion of the loop, and conus formed from the greater part 

 of the anterior limb. The dilatation of the walls of the several 

 chambers is not uniform. In the case of the atrial wall the dilata- 

 tion is most marked dorsally and more especially laterally : the 

 posterior wall on the other hand lags behind in its growth and the 

 result is that the sinu-atrial and atrioventricular openings remain 

 comparatively close to one another (compare adult condition as 

 shown in Fig. 180). Similarly in the case of the ventricular portion 

 of the heart the increase in size is mainly on the ventral and 

 lateral sides, the dorsal wall lagging behind so that the communica- 

 tion between atrium and ventricle and that between ventricle and 

 conus also remain in close proximity (cf. again Fig. 180). 



After the demarcation of the chambers there come about two 

 important changes in the general form of the heart — the first is the 

 assumption of bilateral symmetry on the part of the ventricle, 

 correlated with a rotation of the ventral side of the ventricle towards 

 the animal's left side. The other consists of a very marked increase 

 in the length of the conus which, owing to the fixation of its two 

 ends, is made to assume the characteristic double flexure already 

 described and illustrated (see also Fig. 179, C). 



Development of Septa in the Heart. — By far the most 

 important feature of the Dipnoan heart, as compared with that of 

 the Elasmobranch, is that now, for the first time, there comes about 

 that separation of the heart into an arterial and a venous half, 

 which is so characteristic of the higher Vertebrates. In Lepidosiren 

 this separation is inaugurated at a relatively early stage of develop- 

 ment (stage 27) — at a time when the cardiac tube, as yet, shows no 

 indication of a division into chambers — by proliferation of the cells 

 of the endocardium on its outer surface. This takes place along a. 

 line which passes along the posterior wall of the U-shaped heart 

 from the left side of the sinu-atrial opening, through the auricular 

 canal down towards the apex of the cardiac loop. As this prolifera- 

 tion goes on it causes the endocardium to bulge into the lumen so as 

 to form a prominent ridge traversing the hinder wall of atrium and 



