158 DISCOVERY REPORTS 



parts of the heart has thickened, and in the conus locaUzed swelhngs of this hning 

 indicate the presence of three separate endocardial ridges. 



In stage 24, Fig. 8 d, the conus has undergone an apparently independent flexure. 

 This is not paralleled in the development of the heart of the fowl and it seems to be of 

 the same nature as the N-shaped flexure that develops temporarily in the conus of 

 reptilian embryos (cf. the heart of an embryo crocodile, Kerr, 1919, p. 391)- At this stage 

 the proportions of the conus indicate its importance as a chamber of the heart. At the 

 headward end its cavity is practically divided into two by the approximation of two of 

 the endocardiac ridges. At its base three endocardiac cushions continuous with the 

 ridges form the rudiments of pocket valves that later develop in that region. With regard 

 to other chambers of the heart the interauricular septum begins to form, and between 

 the atrium and ventricle a partial septum is laid down. The atrio-ventricular opening is 

 bounded by two thick endocardiac cushions which serve as atrio-ventricular valves. 



Lastly Fig. 9 c shows the side view of the heart of a penguin embryo at stage 31 . This 

 is comparable with a fowl of 8 days' incubation and represents at least four full days' 

 advance beyond the heart last described. 



In the interim the septa of the conus, the ventricle, and the atrium pass through the 

 chief phases of their development and the heart attains practically to its adult condition. 



The conus septum isolates the pulmonary from the aortic cavities. At the same time 

 the muscular conus wall begins to be incised along the line of the internal septum and 

 the separation between the pulmonary and aortic roots commences. In the adult heart 

 the entrance to the conus from the ventricle is guarded by a circle of three pocket 

 valves, and a similar circle of three pocket valves also occurs at the entrance to the 

 pulmonary artery. The relation of these valves to the original three endocardiac ridges is 

 therefore a matter of considerable interest. In the fowl (LiUie, 1 908, p. 352) the aortic and 

 pulmonary trunks each receive one of the three primary ridges. The remaining primary 

 ridge splits : one half passes into the aortic trunk and the other half into the pulmonary 

 trunk. Provision is thus made for four of the future pocket valves. The other pair of 

 valves — the third valve in each trunk — arise from specially developed secondary endo- 

 cardiac ridges. In penguin embryos the course of development of the conus valves is 

 identical with that described for the fowl. 



With regard to the atrial and ventricular septa, they are discontinuous from the very 

 beginning. The atrial septum originates in the myocardium, forming the inner surface 

 of the atrial wall, and the ventricular septum is derived from that of the ventricle. Thus 

 the latter develops in association with trabeculae and is at first a meshwork of muscular 

 fibres. As the septa grow in length the atrio-ventricular opening constricts into two and 

 they finally unite with the bridge of tissue that partitions this opening. In this way the 

 division of the heart into four quite separate compartments takes place, the auricles and 

 ventricles come into being, and the heart takes on the characteristic appearance seen in 

 stage 31, Fig. 9 c. 



One further point remains to be mentioned. In the complete heart the auriculo- 

 ventricular openings are both provided with important valves. The pair on the right side 



