THE EARLY EMBRYOLOGY OF THE MOUSE 



45 



a groove which is continuous anteriorly and posteriorly with the fore- and 

 hind-guts. This groove is the mid-gut (Fig. 20A). The two sheets of 

 splanchnopleure rapidly draw closer together (Fig. 20B), and at the nineteen 

 somite stage, which may be reached as early as 8^4 days, have fused distally 

 to form a closed tube. 



The heart. — It will be remembered that in 7)2 day embryos there is a 

 small region of mesoderm anterior to the fore-gut (Fig. 15). This forms the 

 base of a U of which the two lateral sheets of mesoderm form the sides. 

 Within this U the coelom develops and is, therefore, itself U-shaped. The 

 base of the U, and the two sides approximately as far caudad as the second 



Fig. 24. — Transverse sections showing the turning of the embryo. A. 8 days i hour, 

 7 somites. B. 8 days 10 hours, 10 somites. (X50.) 



pair of somites, contain that portion of the coelom which ultimately encloses 

 the heart and which, therefore, is known as the pericardial coelom (Fig. 22). 

 The curved shape of the pericardial coelom in cross section in Fig. 22 

 should not be confused with the U-shape of the pericardial coelom as a whole. 

 The heart is derived from the splanchnic mesoderm which forms the 

 ventral wall of the pericardial coelom (Fig. 29). In five somite embryos this 

 mesoderm has differentiated into two layers. Adjacent to the pericardial 

 coelom is a thick, continuous layer, known as the epi-myocardium because 

 it will give rise both to the heavy muscular layer of the heart wall (myo- 

 cardium) and to its outer covering (epicardium). Between the epi-myo- 

 cardium and the underlying entoderm are a number of irregular cavities 

 which later fuse to form the cavity of the heart. The lining of these cavities 

 is the endocardium. 



