CIRCULATORY SYSTEM OF THE GRASS-SNAKE. 615 



mesenteric veins, and the anterior is continued as the short 

 common stem (truncus arteriosus) of the first pair of branchial 

 arches. It soon bends towards the right, and as the two ends 

 remain in approximately the same position, while the tube itself 

 grows longer, it is forced to take on a curved form. In this 

 twisted condition three portions can be distinguished : first, a 

 posterior part running from the union of the omphalo-mesenteric 

 veins, close to which now open the paired ductus cuvieri, 

 ventrally and towards the left ; secondly, a median part situated 

 ventrally and running obliquely from left to right ; and thirdly, 

 an anterior part running from the right to the median line, where 

 it bends sharply dorsally before passing over into the truncus 

 arteriosus, from which, by this time, two pairs of branchial arches 

 are given off. 



Grooves appear on the posterior part of the tube which indicate 

 the divisions between sinus venosus and atrium and between 

 atrium and ventricle. Into the sinus venosus now open the 

 paired umbilical veins. The middle part, afterwards to become 

 the ventricle, becomes dilated ventrally, and as the anterior part 

 also dilates, the two parts are separated by a deep furrow. 

 According to Hochstetter (20) and Langer (29) this anterior 

 part is homologous with the bulbus cordis of the Batrachia. 



The ventricle expands still further and moves caudally, so that 

 the atrium, which has also become dilated, comes to lie close to 

 the bulbus cordis. The further dilatation of the atrium takes 

 place cranially and towards the left, causing it to take up a 

 position on the left side of the bulbus. The constriction between 

 the atrium and ventricle, corresponding to the auricular canal, 

 becomes more marked as these two structures swell out. At this 

 stage, too, the sinus venosus is sharply constricted ofl:' from the 

 atrium and the truncus arteriosus gives oflP the six pairs of 

 branchial arches. Rathke erroneously described only five branchial 

 arches, but this will be referred to again later. 



After this the ventricle gradually assumes its adult shape 

 The base of the bulbus cordis, originally joining the ventricle on 

 the left, moves into an almost mid-ventral position. Spirally 

 twisted grooves appear between the branchial ai-ches, now reduced 

 to three in number, and extend downwards over the bulbus. 

 The atrium now gives off another dilatation, but this time to the 

 right, and consequently the truncus arteriosus and bulbus cordis 

 lie in a deep groove between the outgrowths of the atrium. 

 These two dilatations are the definitive auricles, and already 

 the sinus venosus, which lies in the atrio- ventricular sulcus on the 

 dorsal side of the heart, opens into the one on the right. 



The groove between the bulbus and the ventricle gradually 

 disappears, and ultimately the proximal part of the bulbus 

 becomes incorporated with the latter, while its distal portion 

 becomes assimilated to the truncus arteriosus. The spiral 

 grooves filially extend over the whole of the so-formed truncus, 

 and they indicate its internal division into three parts by the 



