THE LATER DEVELOPMENT OF THE FROG 



175 



the general body cavity, though we shall see that later it be- 

 comes completely closed off. Dorsally there is no definite 

 coelomic space in this, the pharyngeal region. The pericardial 



FIG. 61. Sections showing the formation of the heart in the frog. A-D. 

 Series of transverse sections through corresponding regions of a series of embryos 

 of R. temporaria. After Brachet. E. F. Sections through the same region in 

 older embryos of R. sylvatica. A. 2.6 mm. embryo. Mesoderm approaching 

 the mid-line; endothelium appearing. B. Older embryo of same length as A. 

 C. 3 mm. embryo showing enlargement of pericardial cavity and the begin- 

 ning of the folding of the somatic mesoderm. D. 3.2 mm. embryo. Endothelial 

 cells becoming arranged in the form of a tube. E. Embryo of about 3 mm. 

 F. Embryo of 5-6 mm. Heart tube established; dorsal mesocardium still 

 present, dm, Dorsal mesocardium; e, cardiac endothelial cells; en, endoderm; 

 g, wall of gut (pharynx); p, pericardia! cavity; so, somatic layer of mesoderm; 

 sp, splanchnic layer of mesoderm. 



wall and the muscular wall of the heart are derived from the 

 lateral plate mesoderm, while the inner lining of the heart, the 

 endothelium, is derived from scattered mesoderm cells . lying 



