THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 545 



Fig. 299. Three diagrams to 

 illustrate the formation of 

 the heart in the Chick. 



.', Xeural tube; m, mesen- 

 chyma of the head ; d, in- 

 testinal cavity ; df, folds 

 of the intestinal plate 

 [splanchnopleure], in which 

 the endothelial sacs of the 

 heart are established ; h, en- 

 dothelial sac of the heart ; 

 ch, chorda ; Ih, body- 

 cavity ; ale, outer, ik, inner 

 germ-layer ; 'nilc 1 , parietal 

 middle layer ; ink", visceral 

 middle layer, from the 

 thickened portion of which 

 the musculature of the 

 heart is developed ; dn, in- 

 testinal suture, in which 

 the two intestinal folds are 

 fused ; db, part of the ento- 

 blast which has become de- 

 tached from the epithelium 

 of the cephalic portion of 

 the intestine at the intes- 

 tinal suture and lies on 

 the yolk ; + dorsal meso- 

 cardium ; * ventral meso- 

 cardium. 



A, The youngest stage shows 

 the infolding of the splanch- 

 nopleure, by means of which 

 the cephalic part of the in- 

 testine is formed. In the 

 angles of the intestinal folds 

 the two endothelial sacs of 

 the heart have been esta- 

 blished between the inner 

 germ-layer and the visceral 

 middle layer. 



B, Somewhat older stage. 

 The two folds (A df) have 

 met in the intestinal suture 

 (dn), so that the two endo- 

 thelial sacs of the heart 

 lie close together in the 

 median plane below the 

 head-gut. 



C, Oldest stage. The part of 

 the entoblast which lines 

 the head-gut (<?) has become 

 separated at the intestinal 

 suture (B dn) from the re- 

 maining part of the ento- 

 blast, which (J6) lies upon 



ak 



d m 



n 



ik Ih mk- h df 



d 



B 



n 



mk" h d 



Ih 



I 



n 



h db h mk y lit 



the yolk, so that the two endothelial sacs of the heart are in contact ; they subsequently fuse. 

 They lie in a cardiac suspensorhun formed by the visceral middle layers, the mesocardium, on 

 which one can distinguish an upper [dorsal] and an under part mesocardium superius(-f )and 

 inferius (*). By means of this mesocardium the primitive body-cavity is temporarily divided 

 into two portions. 



35 



