DEVELOPMENT OF PETROMYZON FLUVIATILIS. 189 



The cavity in which the corpuscles lie in great numbers is subse- 

 quently shut off by the mesoblast as it grows downwards and 

 becomes the subintestinal vein. It is from the first continuous 

 with the posterior end of the heart, and the corpuscles soon 

 pass from it into that organ. From the first appearance of 

 the heart in the ventral mesentery its walls have been double ; 

 the splanchnopleure having split into two layers, of these the 

 outer is at first much the thicker consisting of cubical cells ; 

 the inner layer is composed of comparatively flattened cells. 

 The heart at first is a straight tube of the same length as the 

 section of the body cavity in which it lies. Very soon, how. 

 ever, it increases in length, and thus becomes slightly twisted ; 

 at the same time two constrictions appear, dividing it into three 

 chambers. The most posterior of these is the sinus venosus ; 

 it is directly continuous with the space in which the corpuscles 

 are developing. By this time this space has acquired definite 

 walls by the downgrowth of the mesoblast in this region, and 

 it may now be spoken of as the subintestinal vein. 



The liver which developes as a ventral outgrowth of the 

 intestine first makes its appearance in this space, and when the 

 latter gets closed off" as a vein, the liver has become a branched 

 gland projecting into it, so that the blood returning from the 

 alimentary canal passes between the tubuli of the liver. Thus, 

 from the very first an hepatic portal system is present. The 

 tubuli of the liver do not appear to have any continuous meso- 

 blastic coating, though here and there a flattened cell can be 

 distinguished in the outside of a tubule. 



The venous sinus communicates by a narrow opening with 

 the auricle or second chamber of the heart. This in its turn 

 opens by a similar narrow opening into the ventricle. This 

 latter opening is guarded by a pair of valves, which appear by 

 the tenth day after hatching ; they effectually prevent any 

 regurgitation of the blood into the auricle. The walls of the 

 ventricle have undergone a considerable change. From the 

 cells of the inner lining a number of branched muscle-cells 

 have been developed (fig. 36). These cells stretch across the 

 cavity of the ventricle from side to side, and fuse and anas- 



