THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 589 



2. The second type is to be derived from the first, and is explain- 

 able as an adaptation to the great size of the yolk, in that the heart 

 is established at a time when the splanchnopleure is still spread out 

 flat upon the yolk and is not yet folded together to form the head- 

 gut. 



3. The cells which are united to form the endothelium of the 

 heart are split off from a proliferating, thickened place of the 

 entoderm. 



4. The heart is first established in all Vertebrates in the cervico- 

 cephalic region behind the last visceral arch. 



5. The posterior or venous end of the single cardiac tube receives 

 the blood from the body through the omphalomesenteric veins ; the 

 anterior or arterial end gives off the blood to the body through the 

 truncus arteriosus. 



6. In the ainniotic Vertebrates the single cardiac sac is converted 

 by a series of metamorphoses (1) by flexures, constrictions, and 

 changes of position, and (2) by the formation of partitions inside of 

 it into a heart composed of two ventricles and two atria. 



7. The straight sac assumes the form of a letter 8. 



8. The venous portion of the S comes to lie more dorsal, the 

 arterial more ventral ; the two are marked off from each other by a 

 constriction, the auricular canal, and are now to be distinguished 

 as atrium and ventricle. 



9. The venous portion or the atrium forms lateral evaginations, 

 the auricles of the heart, which surround from behind the truncus 

 arteriosus. 



10. The formation of partitions, by which atrium, ventricle, and 

 truncus arteriosus are divided into right and left halves, begins at 

 three different places. 



(a) First of all, the atrium is divided by an atrial partition into 



a right and a left half ; but the separation is incomplete, 

 since there exists a passage in the partition, the foramen 

 ovale, which remains open up to the time of birth. 



(b) By its downward growth the atrial partition reaches the 



auricular canal (septum intermedium of His) and divides 

 the opening in it into a right and left ostium atrioven- 

 triculare. 



(c) The ventricle is divided into right and left halves by a 



partition (septum ventriculi) beginning at the apex of 

 the heart ; the division is also indicated externally by the 

 sulcus inter vent ricularis. 



