CHANGES IN THE OVUM. 



97 



several species, while it is as persistent in others ; varying, of course, 

 with the duration of the vesicle itself. 



The heart, from being merely a cylindrical tube, undergoes considerable 

 modifications before it arrives at its complete development, passing 

 through all the different forms which characterize the organ in the 

 various vertebrate animals. The first change consists in an inflexion, by 

 which the inferior part becomes the superior ; then it dilates at three dis- 

 tinct points : one anterior and superior, at the origin of the aorta, named 

 the aortic bulb ; a middle one, the ventricular cavity ; and a posterior, the 

 auricular cavity The constriction between the auricle and ventricle is 

 named nailer's passage (detroit). 



These two cavities are single only for a brief period ; the ventricular 



I 



Fig. 44. 

 FcETAL Circulation in a Transition State. 



a bcJ, Circle 



c .. n, J Envelope resulting from the Fusion of the Vitelline Membrane, External Layer 

 of the Bastodermic Vesicle, and the Transformation of the Allantois ; c, The Diminishing 

 Umbihcal Vesicle: ^, Cephalic Portion of the Embryo; ^., Caudal Portion; e, Ventricle 0I 

 the Heart ,,/; Auricle *f the Heart ; /, Aorta, forming the Aortic Arches ; //, Trunk Represent- 

 ing the Thoracic Aorta ;^, Vessel which becomes the Anterior Vena Cava; k. Vena Azygos ; 

 I, Confluents of the two Veins ^ and k ; m, Confluent of all the Veins at their Entrance into the 



Auricle of the Heart; «, Vessel Resulting from the Union of the^Allantoid'Ve'i'ns"/ A "and tlie 

 Omphalo-Meseraic Vein g; o Posterior Vena Cava; / /, Allantoid Veins; /omphalc^ 



Meseraic or Umbilical Vesicle Vein • - »—'-- ...i-^'^'. . ...» y... \»v".<»'"- 



t, Omphalo-Meseraic Artery. 



Posterior or Abdominal Aorta ; j s, Allantoid Arteries ; 



first divides into two— a division marked externally by a furrow which is 



visible on the surface of the heart of a foetal lamb at the nineteenth day, 



and the twenty fifth in the foal. This furrow corresponds to an inter- 



[ventricular septum which arises insensibly at the bottom of the ventricles, 



jand reaches the auricles, where it concurs to form the auriculo-ve^itricular 



lorijices. The borders of the openings are furnished with a slightly salient 



lip, which at a later stage develops, and constitutes the mitral and 



