564 EMBRYOLOGY. 



takes place independently of the development of a partition in the 

 ventricle, beginning as it does at first above and advancing from 

 there downwards. Finally the aortic septum penetrates also into 

 the cavity of the ventricle itself (tig. 310 B s and ks), there unites 

 with the independently developed ventricular partition, furnishes 

 the part known as pars membranacea (Oi), and thus completes the 

 separation of the vessels leading out from the heart, the aorta falling 

 to the lot of the left ventricle, the art. pulmonalis to the light. 



The pars membranacea indicates therefore in the finished heart 

 the place at which the separation between the right and left halves 

 of the heart is completed (fig. 310 B Oi}. "It is, as it were, the 

 keystone in the final separation of the primitive simple cardiac sac 

 into the four secondary cardiac cavities, as they are formed in Birds 

 and Mammals " (Ross). From a comparative-anatomical point of 

 view this place presents a special interest 

 from the fact that in Reptiles there exists 

 here a permanent opening between the 

 two ventricles, the foramen Pannizzae. 



Even before the division of the truncus 

 Fig. 312. Diagram of the ar- arteriosus, the semilunar valves have become 



rangement of the arterial T 7 . , ,, . , . . r 



valves. From GEGENBAUR. established as four ridges, consisting of 

 A, undivided tmncus arteriosus gelatinous tissue with a covering of endo- 



with four fundaments of . . . . . 



valves. B, Division into pui- thelium, at the contracted place which is 

 monaiis ( P ) and aorta (a), designated as the /return Halleri. Two of 



each of which possesses three ,-,-,', PIT- 



valves. them are halved at the time or the divi- 



sion of the truncus into aorta and art. 



pulmonalis. For each vessel, therefore, there are now three ridges, 

 which, owing to a shrivelling of the gelatinous tissue, assume the 

 form of pockets. Their arrangement, to which GEGENBAUR has called 

 attention, is intelligible from their method of development, as the 

 accompanying diagram (fie. 312) shows. "By the division of the 

 originally single bulbus arteriosus (A) into two canals (), the 

 nodule-like fundaments of the four original valves are distributed 

 in such a manner that the anterior [ventral] one and the anterior 

 halves of the two lateral ones fall to the anterior arterial trunk 

 (pulmonalis), the posterior and the posterior halves of the lateral 

 ones to the posterior arterial trunk (aorta)." 



Finally, as regards the atrium, it is to be said that the sinus 

 venosus, mentioned at p. 558, the mouth of the pulmonary vein, and 

 the foramen ovale undergo important alterations. 



The sinus venosus disappears as an independent structure, since it 



