

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 



sp 



en 



so 



E 



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



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



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



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



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



7. 3 mm. embryo showing enlargement of pericardial cavity and the begin- 



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



11s becoming arranged in the form of a tube. E. Embryo of about 3 mm. 



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

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



wall of gut (pharynx); p, pericardial cavity; so, somatic layer of mesoderm; 



splanchnic layer of mesoderm. 



'-all and the muscular wall of the heart are derived from the 

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

 idothelium, is derived from scattered mesoderm cells lying 





