in ike Foetus of Vertebrated Animals. 809 



gelatinous consistence. As evolution proceeds, its texture be- 

 comes more compact ; it now takes the form of a sac, pointed 

 at the two extremities: the posterior extremity becomes con- 

 nected with the venous trunk which collects the blood from 

 the anterior surface of the yolk. At a later period, the rudi- 

 mentary heart is divided into two compartments by a knotch 

 in the middle (Fig. 3). The posterior of these compartments 

 (r, Fig. 4). communicating with the vena cava and veins from 

 the yolk, is the auricle ; its form is oval and more regular than 

 that of the other compartment, and it is placed in the direc- 

 tion of the body of the fish. The anterior compartment (*), or 

 ventricle, projects from the body of the foetus, and communi- 

 cates with the posterior auricular part by a short and narrow 

 canal : the cavity of the ventricle is widest posteriorly : at its 

 anterior extremity it is connected by a canal with an oval dila- 

 ted part, which afterwards forms the bulb of the aorta (<). At 

 first the cavities of the heart appear scarcely to differ from one 

 another in their texture. Very soon, however, the parietes of 

 the ventricle become perceptibly thicker, and this cavity in con- 

 sequence assumes a white appearance during its contraction : 

 the parietes of the auricle, on the other hand, remain very thin, 

 so as to allow the red colour of the blood to shine through them 

 (Fig. 5). As the development of the heart proceeds, the veins on 

 the surface of the yolk become more apparent. The course of 

 the blood can then be traced with ease. A branch springing 

 from the vena cava inferior, about the middle of the abdomen, 

 (Fig. 7, v), is ramified on the posterior surface of the yolk, and 

 communicates, by its capillai-y vessels, with the small branches 

 of another vein A/, which is ramified on the anterior surface of 

 the yolk, and which, joining again the vena cava near the head, 

 conveys the blood which has passed over the yolk into the au- 

 ricle of the heart. Before entering the heart, the venae cavae 

 form posteriorly, by their dilatation, two sacs, which afterwards 

 become the venous sinuses or appendages to the auricle. 



As development proceeds, the auricle increases principally in 

 its width ; the auricular canal shortens itself relatively ; the 

 ventricle increasing in size and strength acquires the form of 

 a three-sided pyramid ; the bulb of the aorta becomes stronger 

 and wider than before. The relative position also of the auricle 

 and ventricle is changed. As the sac of the yolk decreases. 



