THE HEART 157 



markable that while ossification in the cartilages is only beginning, the clavicle, a com- 

 paratively unimportant bone, should be so far advanced. 



THE HEART 



The early development of the heart of the penguin follows the usual course. It is laid 

 down as a simple tube of splanchnic mesoderm enclosing a thin endocardiac lining. At a 

 stage comparable with a fowl embryo of about 46 hours' incubation, stage 12, the 

 cardiac tube seen from the ventral side is a simple bladder-like vessel, narrowing an- 

 teriorly and slightly bent upon itself (Fig. 8 a). By stage 13 the tube has taken on the 

 familiar S-shaped bend of the heart of a forty-hour fowl (cf. Hochstetter, 1906, p. 38). 

 The flexure of the tube is due to its rapid increase in length within the confined space of 

 the pericardiac cavity (cf. Kerr, 1919, p. 370), and the increase in diameter of the hinder 

 end of the tube, where it receives the great vitelline veins, is reflected in the dominant 

 appearance of this part of the heart when it is seen from the ventral surface (Fig. 8 b). 



1 



k 



KJ 



ail 



a b c d 



Fig. 8. The heart, ventral view, a, stage 12; b, stage 13; c, stage 18; d, stage 24. In d the head is cut 

 away to the level of the optic lobes in order to expose the heart, au, atrium; co, conus arteriosus; v, ven- 

 tricle. 



In side view, the flexure is plainer (Fig. 9 a). Sections of the heart at this stage show 

 that in their active proliferation the cells of the endocardium keep pace with the growth 

 of the myocardium. They are more active in growth in the localities where the heart is 

 least afl'ected by the flexure. 



The next stage in the development of the heart, illustrated in Figs. 8 c and 9 b, was 

 drawn from an embryo at about stage 18. It compares therefore with the heart of a fowl 

 of 67 hours' incubation. In the intervening period between this stage and stage 13 the 

 heart has undergone profound modification. The centre of the cardiac tube has grown 

 extensively, forming the ventricle, and its fixed anterior end has come prominently into 

 view in the mid-ventral line and forms the conus arteriosus (Fig. 8 c, co). The other end 

 of the cardiac tube, the atrial part, remains on the dorsal side of the ventricle. Its 

 cavity is not divided until a later stage, but the opening into it from the vitelline veins is 

 guarded by a well-developed valve. The endocardium between the atrial and ventricular 



3-2 



