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 

 ccelomic space in this, the pharyngeal region. The pericardial 



E 



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, pericardial 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 

 cn<lot]teUum, is derived from scattered mesoderm cells lying 



