546 EMBRYOLOGY. 



surround a small cavity, the primitive cardiac cavity (A). These cells 

 assume more of an endothelial character. While the intestinal folds 

 grow toward each other, the two endothelial tubes become enlarged 

 and push the thickened part of the visceral middle layer before them, 

 so that the latter forms a low, ridge-like elevation into the primitive 

 body-cavity. In the embryos of higher Vertebrates also, just as in 

 the Amphibia, this stretches forward into the embryonic fundament 

 as far as the last visceral arch, and has here received the special name 

 of neck-cavity or parietal cavity. 



In older embryos (fig. 299 J5) the edges of the two folds have met 

 in the median plane, and consequently the two cardiac tubes have 

 moved close together. A process of fusion then takes place between 

 the corresponding parts of the two intestinal folds. 



First the entoblastic layers fuse, and in this way is produced 

 (fig. 299 B] beneath the chorda dorsalis (ch) the cavity of the head-gut 

 (d), which then detaches itself from the remaining part of the ento- 

 blast (fig. 299 C db) ; the latter is left lying on the yolk and becomes 

 the yolk-sac. Under the cavity of the head-gut the two cardiac 

 sacs have come close together, so that their cavities are separated 

 from each other by their own endothelial walls only. By the break- 

 ing through of these there soon arises from them (K) a single cardiac 

 tube. . On the side toward the body-cavity this is covered by the 

 visceral middle layer (m& 2 ), the cells of which are distinguished in 

 the region of the fundament of the heart by their great length and 

 furnish the material for the cardiac musculature, while the inner 

 endothelial membrane becomes only the endocardium. 



The whole fundament of the heart lies, as in the Amphibia, in a 

 ventral mesentery, the upper [dorsal] part of which, extending from 

 the heart to the head-gut (fig. 299 G +), can here also be called the 

 dorsal suspensory of the heart or mesocardium posterius, and the 

 lower [ventral] part (*) mesocardium aiiterius. In the Chick, when 

 the cardiac tube begins to be elongated and bent into an S-shaped 

 form, the mesocardium anterius quickly disappears. 



Similar conditions are furnished by cross sections through Rabbit 

 embryos 8 or 9 days old. In the latter the paired fundaments of the 

 heart are indeed developed still earlier and more distinctly than in the 

 Chick, even at a time when the entoderm is still spread out flat and 

 has not yet begun to be infolded. Upon cross sections one sees 

 (fig. 301), in a small region at some distance from the median plane, 

 the splanchnopleure separated from the somatopleure by a small 

 fissure (ph), which is the front end of the primitive body-cavity. At 



