274 OUTLINES OF CHORDATE DEVELOPMENT 



jor.str. 



FIG. 106. Chick embryo with seven pairs 

 of somites (about twenty-five hours). Dor- 

 sal view. From Lillie (Development of the 

 Chick), a.c.s., Anterior cerebral suture; ceph. 

 Mes., cephalic mesoderm; F.G., fore-gut; N'ch., 

 notochord; n. T., neural tube; op. Ves., optic 

 vesicle; Pr'a., proamnion; pr. str., primitive 

 streak; s. 2, s. 7, second and seventh somites; 

 V.o.m., omphalomesenteric vein. 



together to form a me- 

 dian, thin-walled tube. 

 This becomes the endo- 

 thelial lining of the heart ; 

 the muscular wall of the 

 heart is formed by the 

 addition of an external 

 layer of mesoderm; for 

 the splanchnic mesoderm 

 of each side forms a fold 

 around the endothelial 

 rudiment (Fig. 105, B). 

 These folds then ap- 

 proach and fuse across 

 the mid-line, both above 

 and below the endothe- 

 lial tube. For a brief 

 period after their fusion 

 they remain in the mid- 

 line forming a dorsal meso- 

 cardium, connecting the 

 heart with the tissues just 

 beneath the fore-gut, and 

 a ventral mesocardium, 

 extending from the heart 

 to the splanchnopleure. 

 The ventral mesocar- 

 dium breaks away almost 

 immediately after its for- 

 mation, while the dorsal 

 mesocardium remains for 

 a time, but then disap- 

 pears, except at the an- 

 terior and posterior ex- 

 tremities of the heart. 

 The heart is then left as 

 a short median tube con- 



